Semiconductor device, display device including semiconductor device, display module including display device, and electronic device including semiconductor device, display device, and display module

ABSTRACT

A semiconductor device including a transistor and a wiring electrically connected to the transistor each of which has excellent electrical characteristics because of specific structures thereover is provided. A first conductive film, a first insulating film over the first conductive film, a second conductive film over the first insulating film, a second insulating film over the second conductive film, a third conductive film electrically connected to the first conductive film through an opening provided in the first insulating film and the second insulating film, and a third insulating film over the third conductive film are provided. The third conductive film includes indium, tin, and oxygen, and the third insulating film includes silicon and nitrogen and the number of ammonia molecules released from the third insulating film is less than or equal to 1×10 15  molecules/cm 3  by thermal desorption spectroscopy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/659,792, filed Mar. 17, 2015, which claims the benefit of a foreignpriority application filed in Japan as Serial No. 2014-057528 on Mar.20, 2014, both of which are incorporated by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a semiconductordevice including an oxide semiconductor and a display device includingthe semiconductor device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, the presentinvention relates to a process, a machine, manufacture, or a compositionof matter. In particular, the present invention relates to asemiconductor device, a display device, a light-emitting device, a powerstorage device, a storage device, a driving method thereof, or amanufacturing method thereof.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A semiconductor element such as a transistor, asemiconductor circuit, an arithmetic device, and a memory device areeach an embodiment of a semiconductor device. An imaging device, adisplay device, a liquid crystal display device, a light-emittingdevice, an electro-optical device, a power generation device (includinga thin film solar cell, an organic thin film solar cell, and the like),and an electronic device may each include a semiconductor device.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor usinga semiconductor thin film formed over a substrate having an insulatingsurface (also referred to as a field-effect transistor (FET) or a thinfilm transistor (TFT)). Such transistors are applied to a wide range ofelectronic devices such as an integrated circuit (IC) and an imagedisplay device (display device). A semiconductor material typified bysilicon is widely known as a material for a semiconductor thin film thatcan be used for a transistor. As another material, an oxidesemiconductor has been attracting attention (see Patent Document 1).

For example, as a transistor including an oxide semiconductor film, atransistor is disclosed in which the numbers of hydrogen molecules andammonia molecules which are released from a nitride insulating filmprovided over the transistor are reduced and a change in electricalcharacteristics is suppressed (see Patent Document 2).

In recent years, with increased performance and reductions in the sizeand weight of electronic devices, demand for a display device in which adriver circuit is formed so that miniaturized transistors, connectionwirings, or the like are integrated with high density, and the drivercircuit and the display device are provided on the same substrate hasrisen.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165529-   [Patent Document 2] Japanese Published Patent Application No.    2014-030002

SUMMARY OF THE INVENTION

As a wiring connected to a transistor (also referred to as a leadwiring), a multi-layer structure is preferred to a single-layerstructure because the wirings can be integrated with high density. Inthe case of a wiring having a multi-layer structure, it is preferable touse a conductive film which is formed through steps of processing thesame conductive film as conductive films used for a gate electrode, asource electrode, or a drain electrode of the transistor or a pixelelectrode electrically connected to the transistor, in which case themanufacturing cost can be reduced because the number of steps (thenumber of masks) can be reduced.

For example, in the case where a transparent conductive film functioningas a pixel electrode is used for the wiring connected to a transistor(such as a lead wiring), the wirings can be integrated with highdensity. When used for a lead wiring or the like, however, thetransparent conductive film might be corroded during operation in ahigh-temperature and high-humidity environment (e.g., operation at atemperature of 60° C. and a humidity of 95%). When included in a displaydevice, a semiconductor device having such a wiring decreases the yieldof the display device because of corrosion of the wiring.

In addition, when the transistor includes an oxide semiconductor film ina semiconductor layer and a protective film is formed over the wiring toprevent its corrosion, entry of moisture or the like released from theprotective film into the oxide semiconductor film might changeelectrical characteristics of the transistor.

In view of the above problems, an object of one embodiment of thepresent invention is to provide a semiconductor device including atransistor and a wiring electrically connected to the transistor each ofwhich has excellent electrical characteristics because of specificstructures thereover.

Another object of one embodiment of the present invention is to providea semiconductor device with high productivity. Another object of oneembodiment of the present invention is to provide a semiconductor devicethat is suitable for miniaturization. Another object of one embodimentof the present invention is to provide a semiconductor device includingan oxide semiconductor with favorable electrical characteristics.Another object of one embodiment of the present invention is to providea highly reliable semiconductor device including an oxide semiconductorin which a change in the electrical characteristics is suppressed.Another object of one embodiment of the present invention is to providea novel semiconductor device. Another object of one embodiment of thepresent invention is to provide a novel display device.

Note that the description of the above object does not disturb theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Objects other than theabove objects will be apparent from and can be derived from thedescription of the specification and the like.

One embodiment of the present invention is a semiconductor deviceincluding a first conductive film, a first insulating film over thefirst conductive film, a second conductive film over the firstinsulating film, a second insulating film over the second conductivefilm, a third conductive film electrically connected to the firstconductive film through an opening provided in the first insulating filmand the second insulating film, and a third insulating film over thethird conductive film. The third conductive film includes indium andoxygen, and the third insulating film includes silicon and nitrogen andthe number of ammonia molecules released from the third insulating filmis less than or equal to 1×10¹⁵ molecules/cm³ by thermal desorptionspectroscopy.

Another embodiment of the present invention is a semiconductor deviceincluding a first conductive film, a first insulating film over thefirst conductive film, an oxide semiconductor film over the firstinsulating film, a pair of second conductive films electricallyconnected to the oxide semiconductor film, a second insulating film overthe oxide semiconductor film and the pair of second conductive films, athird conductive film electrically connected to the first conductivefilm through an opening provided in the first insulating film and thesecond insulating film, and a third insulating film over the thirdconductive film. The third conductive film includes indium and oxygen,and the third insulating film includes silicon and nitrogen and thenumber of ammonia molecules released from the third insulating film isless than or equal to 1×10¹⁵ molecules/cm³ by thermal desorptionspectroscopy.

In any of the above embodiments, it is preferable that the thirdconductive film further include tin and silicon.

In any of the above embodiments, it is preferable that the oxidesemiconductor film include oxygen, In, Zn, and M (M is Ti, Ga, Y, Zr,La, Ce, Nd, or Hf), and it is preferable that the oxide semiconductorfilm include a crystal part and that the crystal part have c-axisalignment.

Another embodiment of the present invention is a display deviceincluding the semiconductor device according to any one of the aboveembodiments, and a display element. Another embodiment of the presentinvention is a display module including the display device and a touchsensor. Another embodiment of the present invention is an electronicdevice including the semiconductor device according to any one of theabove embodiments, the display device, or the display module; and atleast one of an operation key and a battery.

According to one embodiment of the present invention, a semiconductordevice including a transistor and a wiring electrically connected to thetransistor each of which has excellent electrical characteristicsbecause of specific structures thereover, or a semiconductor devicehaving excellent productivity can be provided. According to oneembodiment of the present invention, a semiconductor device that issuitable for miniaturization can be provided. According to oneembodiment of the present invention, a semiconductor device including anoxide semiconductor can be provided with favorable electricalcharacteristics. According to one embodiment of the present invention, ahighly reliable semiconductor device including an oxide semiconductor inwhich a change in the electrical characteristics is suppressed can beprovided. According to one embodiment of the present invention, a novelsemiconductor device can be provided. According to one embodiment of thepresent invention, a novel display device can be provided.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a top view and a cross-sectional view illustratingone embodiment of a semiconductor device.

FIGS. 2A and 2B are a top view and a cross-sectional view illustratingone embodiment of a semiconductor device.

FIGS. 3A to 3C are a top view and cross-sectional views illustrating oneembodiment of a semiconductor device.

FIGS. 4A to 4C are a top view and cross-sectional views illustrating oneembodiment of a semiconductor device.

FIGS. 5A to 5C are a top view and cross-sectional views illustrating oneembodiment of a semiconductor device.

FIGS. 6A to 6C are a top view and cross-sectional views illustrating oneembodiment of a semiconductor device.

FIGS. 7A to 7D are cross-sectional views each illustrating oneembodiment of a semiconductor device.

FIGS. 8A and 8B are band diagrams.

FIGS. 9A to 9C are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 10A and 10B are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 11A and 11B are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 12A and 12B are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 13A to 13D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 14A to 14C are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 15A and 15B are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 16A to 16D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 17A to 17D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 18A to 18D are Cs-corrected high-resolution TEM images of a crosssection of a CAAC-OS and a cross-sectional schematic view of a CAAC-OS.

FIGS. 19A to 19D are Cs-corrected high-resolution TEM images of a planeof a CAAC-OS.

FIGS. 20A to 20C show structural analysis of a CAAC-OS and a singlecrystal oxide semiconductor by XRD.

FIGS. 21A and 21B show electron diffraction patterns of a CAAC-OS.

FIG. 22 is a top view illustrating one embodiment of a display device.

FIG. 23 is a cross-sectional view illustrating one embodiment of adisplay device.

FIG. 24 is a cross-sectional view illustrating one embodiment of adisplay device.

FIGS. 25A to 25C are a block diagram and circuit diagrams illustrating adisplay device.

FIG. 26 illustrates a display module.

FIGS. 27A to 27H illustrate electronic devices.

FIG. 28 shows temperature dependence of resistivity.

FIG. 29 shows the numbers of ammonia molecules released in samples inExample.

FIG. 30 is a top view illustrating a sample in Example.

FIGS. 31A and 31B show observation results by an optical micrograph inExample.

FIGS. 32A and 32B show observation results by an optical micrograph inExample.

FIG. 33 shows a change in crystal part of an In—Ga—Zn oxide induced byelectron irradiation.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments will be described with reference to drawings.However, the embodiments can be implemented with various modes. It willbe readily appreciated by those skilled in the art that modes anddetails can be changed in various ways without departing from the spiritand scope of the present invention. Thus, the present invention shouldnot be interpreted as being limited to the following description of theembodiments. In describing structures of the present invention withreference to the drawings, common reference numerals are used for thesame portions in different drawings. Note that the same hatched patternis applied to similar parts, and the similar parts are not especiallydenoted by reference numerals in some cases.

In the drawings, the size, the layer thickness, or the region isexaggerated for clarity in some cases. Therefore, embodiments of thepresent invention are not limited to such a scale. Note that thedrawings are schematic views showing ideal examples, and embodiments ofthe present invention are not limited to shapes or values shown in thedrawings.

Note that in this specification, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

Note that in this specification, terms for describing arrangement, suchas “over” “above”, “under”, and “below”, are used for convenience indescribing a positional relation between components with reference todrawings. Furthermore, the positional relation between components ischanged as appropriate in accordance with a direction in which eachcomponent is described. Thus, there is no limitation on terms used inthis specification, and description can be made appropriately dependingon the situation.

In this specification and the like, a transistor is an element having atleast three terminals of a gate, a drain, and a source. In addition, thetransistor has a channel region between a drain (a drain terminal, adrain region, or a drain electrode) and a source (a source terminal, asource region, or a source electrode), and current can flow through thedrain region, the channel region, and the source region. Note that inthis specification and the like, a channel region refers to a regionthrough which current mainly flows.

Furthermore, functions of a source and a drain might be switched whentransistors having different polarities are employed or a direction ofcurrent flow is changed in circuit operation, for example. Therefore,the terms “source” and “drain” can be switched in this specification andthe like.

Note that in this specification and the like, the expression“electrically connected” includes the case where components areconnected through an “object having any electric function”. There is noparticular limitation on an “object having any electric function” aslong as electric signals can be transmitted and received betweencomponents that are connected through the object. Examples of an “objecthaving any electric function” are a switching element such as atransistor, a resistor, an inductor, a capacitor, and elements with avariety of functions as well as an electrode and a wiring.

Note that in this specification and the like, a “silicon oxynitridefilm” refers to a film that includes oxygen at a higher proportion thannitrogen, and a “silicon nitride oxide film” refers to a film thatincludes nitrogen at a higher proportion than oxygen.

In this specification and the like, the term “parallel” indicates thatthe angle formed between two straight lines is greater than or equal to−10° and less than or equal to 10°, and accordingly also includes thecase where the angle is greater than or equal to −5° and less than orequal to 5°. In addition, a term “perpendicular” indicates that theangle formed between two straight lines is greater than or equal to 80°and less than or equal to 100°, and accordingly includes the case wherethe angle is greater than or equal to 85° and less than or equal to 95°.

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of thepresent invention will be described with reference to FIGS. 1A and 1B,FIGS. 2A and 2B, FIGS. 3A to 3C, FIGS. 4A to 4C, FIGS. 5A to 5C, FIGS.6A to 6C, FIGS. 7A to 7D, FIGS. 8A and 8B, FIGS. 9A to 9C, FIGS. 10A and10B, FIGS. 11A and 11B, FIGS. 12A and 12B, FIGS. 13A to 13D, FIGS. 14Ato 14C, FIGS. 15A and 15B, FIGS. 16A to 16D, and FIGS. 17A to 17D.

<Structure Example 1 of Semiconductor Device>

FIG. 1A is a top view of a semiconductor device of one embodiment of thepresent invention. FIG. 1B is a cross-sectional view taken along adashed dotted line A1-A2 in FIG. 1A. Note that in FIG. 1A, somecomponents of the semiconductor device (e.g., an insulating film) arenot illustrated to avoid complexity. As in FIG. 1A, some components arenot illustrated in some cases in top views of semiconductor devicesdescribed below.

The semiconductor device illustrated in FIGS. 1A and 1B includes aconductive film 104 (also referred to as a first conductive film) over asubstrate 102, an insulating film 106 (also referred to as a firstinsulating film) over the substrate 102 and the conductive film 104, aconductive film 112 (also referred to as a second conductive film) overthe insulating film 106, insulating films 114, 116, and 118 (alsocollectively referred to as a second insulating film) over theconductive film 112, a conductive film 120 (also referred to as a thirdconductive film) electrically connected to the conductive film 104through an opening 142 provided in the insulating film 106 and theinsulating films 114, 116, and 118, and an insulating film 122 (alsoreferred to as a third insulating film) over the conductive film 120.

The insulating film 106 has a stacked-layer structure of an insulatingfilm 106 a and an insulating film 106 b. Note that the structure of theinsulating film 106 is not limited thereto, the insulating film 106 mayhave a single-layer structure or a stacked-layer structure of three ormore layers.

It is preferable that the conductive film 104 be formed through steps ofprocessing the same conductive film as a conductive film used for a gateelectrode of a transistor. It is preferable that the conductive film 112be formed through steps of processing the same conductive film as aconductive film used for a source electrode and a drain electrode of thetransistor. It is preferable that the conductive film 120 be formedthrough steps of processing the same conductive film as a conductivefilm used for a pixel electrode electrically connected to thetransistor. The manufacturing cost can be reduced by thus forming theconductive films 104, 112, and 120 through steps of processing the sameconductive films as the conductive films used for the transistor or theconductive film electrically connected to the transistor.

As illustrated in FIGS. 1A and 1B, the conductive films 104, 112, and120 can be integrated with high density by being formed through steps ofprocessing the same conductive films as the conductive films used forthe transistor or the conductive film electrically connected to thetransistor and having a multilayer structure with the insulating filmspositioned therebetween.

The conductive film 120 contains indium and oxygen. Alternatively, theconductive film 120 contains indium, tin, oxygen, and silicon. Amaterial used for the conductive film 120 can be a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

The insulating film 122 contains silicon and nitrogen. The number ofammonia molecules released from the insulating film 122 is less than orequal to 1×10¹⁵ molecules/cm³ when analyzed by thermal desorptionspectroscopy (TDS).

The insulating film 122 has a function of suppressing entry of moisturefrom the outside. In addition, the insulating film 122 includes a regionfrom which a small number of ammonia molecules are released in the TDSanalysis. With such an insulating film 122, corrosion of the conductivefilm 120 can be suppressed because entry of moisture from the outsideduring operation in a high-temperature and high-humidity environment(e.g., operation at a temperature of 60° C. and a humidity of 95%) issuppressed and the amount of moisture or the number of ammonia moleculesreleased from the insulating film 122 is small. Since the insulatingfilm 122 can suppress entry of moisture from the outside, corrosion ofthe conductive film 104 and the conductive film 112 can also besuppressed. Note that the insulating film 122 may have a single-layerstructure or a stacked-layer structure of two or more layers.

Next, other components of the semiconductor device of this embodimentare described below in detail.

<Substrate>

There is no particular limitation on the property of a material and thelike of the substrate 102 as long as the material has heat resistanceenough to withstand at least heat treatment to be performed later. Forexample, a glass substrate, a ceramic substrate, a quartz substrate, asapphire substrate, or the like may be used as the substrate 102.Alternatively, a single crystal semiconductor substrate or apolycrystalline semiconductor substrate made of silicon, siliconcarbide, or the like, a compound semiconductor substrate made of silicongermanium or the like, an SOI substrate, or the like may be used as thesubstrate 102. Still alternatively, any of these substrates providedwith a semiconductor element may be used as the substrate 102. In thecase where a glass substrate is used as the substrate 102, a glasssubstrate having any of the following sizes can be used: the 6thgeneration (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm),and the 10th generation (2950 mm×3400 mm). Thus, a large-sized displaydevice can be manufactured.

Note that in this specification and the like, a semiconductor device canbe formed using a variety of substrates. The type of a substrate is notlimited to a certain type. As the substrate, a semiconductor substrate(e.g., a single crystal substrate or a silicon substrate), an SOIsubstrate, a glass substrate, a quartz substrate, a plastic substrate, ametal substrate, a stainless steel substrate, a substrate includingstainless steel foil, a tungsten substrate, a substrate includingtungsten foil, a flexible substrate, an attachment film, paper includinga fibrous material, or a base material film can be used, for example. Asan example of a glass substrate, a barium borosilicate glass substrate,an aluminoborosilicate glass substrate, or a soda lime glass substratecan be given. Examples of a flexible substrate, an attachment film, abase material film, or the like are as follows: plastic typified bypolyethylene terephthalate (PET), polyethylene naphthalate (PEN), andpolyether sulfone (PES); a synthetic resin such as acrylic;polypropylene; polyester; polyvinyl fluoride; polyvinyl chloride;polyamide; polyimide; aramid; epoxy; an inorganic vapor deposition film;and paper.

Alternatively, a flexible substrate may be used as the substrate, andthe transistor may be provided directly on the flexible substrate.Further alternatively, a separation layer may be provided between thesubstrate and the transistor. The separation layer can be used when partor the whole of a semiconductor device formed over the separation layeris separated from the substrate and transferred onto another substrate.In such a case, the transistor can be transferred to a substrate havinglow heat resistance or a flexible substrate as well. For the aboveseparation layer, a stack of inorganic films, which are a tungsten filmand a silicon oxide film, or an organic resin film of polyimide or thelike formed over a substrate can be used, for example.

In other words, a semiconductor device may be formed using onesubstrate, and then transferred to another substrate. Examples of asubstrate to which a semiconductor device is transferred are, inaddition to the above substrate over which the semiconductor device canbe formed, a paper substrate, a cellophane substrate, an aramidsubstrate, a polyimide film substrate, a stone substrate, a woodsubstrate, a cloth substrate (including a natural fiber (e.g., silk,cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, orpolyester), a regenerated fiber (e.g., acetate, cupra, rayon, orregenerated polyester), and the like), a leather substrate, and a rubbersubstrate. When such a substrate is used, a semiconductor device withexcellent properties or a semiconductor device with low powerconsumption can be formed, a semiconductor device with high durability,high heat resistance can be provided, or reduction in weight orthickness can be achieved.

<First Conductive Film>

The conductive film 104 can be formed by a sputtering method or the likeusing a metal element selected from chromium (Cr), copper (Cu), aluminum(Al), gold (Au), silver (Ag), zinc (Zn), molybdenum (Mo), tantalum (Ta),titanium (Ti), tungsten (W), manganese (Mn), nickel (Ni), iron (Fe), andcobalt (Co); an alloy including any of these metal elements as itscomponent; an alloy including a combination of any of these elements; orthe like.

Furthermore, the conductive film 104 may have a single-layer structureor a stacked-layer structure of two or more layers. For example, asingle-layer structure of an aluminum film containing silicon, atwo-layer structure in which a titanium film is stacked over an aluminumfilm, a two-layer structure in which a titanium film is stacked over atitanium nitride film, a two-layer structure in which a tungsten film isstacked over a titanium nitride film, a two-layer structure in which atungsten film is stacked over a tantalum nitride film or a tungstennitride film, and a three-layer structure in which a titanium film, analuminum film, and a titanium film are stacked in this order can begiven. Alternatively, an alloy film or a nitride film in which aluminumand one or more elements selected from titanium, tantalum, tungsten,molybdenum, chromium, neodymium, and scandium are combined may be used.

The conductive film 104 can be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

A Cu—X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) may be usedfor the conductive film 104. Use of an Cu—X alloy film enables themanufacturing cost to be reduced because wet etching process can be usedin the processing.

<First Insulating Film>

As the insulating film 106, an insulating layer including at least oneof the following films formed by a plasma enhanced chemical vapordeposition (PECVD) method, a sputtering method, or the like can be used:a silicon oxide film, a silicon oxynitride film, a silicon nitride oxidefilm, a silicon nitride film, an aluminum oxide film, a hafnium oxidefilm, an yttrium oxide film, a zirconium oxide film, a gallium oxidefilm, a tantalum oxide film, a magnesium oxide film, a lanthanum oxidefilm, a cerium oxide film, and a neodymium oxide film. Note that insteadof a stacked-layer structure of the insulating films 106 a and 106 b, aninsulating film of a single layer or three or more layers formed using amaterial selected from the above may be used.

In this embodiment, a silicon nitride film is formed as the insulatingfilm 106 a, and a silicon oxide film is formed as the insulating film106 b.

<Second Conductive Film>

The conductive film 112 can be formed using a material and a methodwhich are similar to those of the conductive film 104.

<Second Insulating Film>

The insulating films 114, 116, and 118 collectively function as aprotective insulating film. The insulating films 114 and 116 containoxygen. Furthermore, the insulating film 114 is an insulating film whichis permeable to oxygen.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 5 nm and less than or equal to 150nm, or preferably greater than or equal to 5 nm and less than or equalto 50 nm can be used as the insulating film 114.

The insulating film 116 is formed using an oxide insulating film thatcontains oxygen in excess of that in the stoichiometric composition.Part of oxygen is released by heating from the oxide insulating filmcontaining oxygen in excess of that in the stoichiometric composition.The oxide insulating film containing oxygen in excess of that in thestoichiometric composition is an oxide insulating film of which theamount of released oxygen converted into oxygen molecules is greaterthan or equal to 1.0×10¹⁹ atoms/cm³, or preferably greater than or equalto 3.0×10²⁰ atoms/cm³ in TDS analysis. Note that the temperature of thefilm surface in the TDS analysis is preferably higher than or equal to100° C. and lower than or equal to 700° C., or higher than or equal to100° C. and lower than or equal to 500° C.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 30 nm and less than or equal to 500nm, or preferably greater than or equal to 50 nm and less than or equalto 400 nm can be used as the insulating film 116.

Furthermore, the insulating films 114 and 116 can be formed usinginsulating films formed of the same kinds of materials; thus, a boundarybetween the insulating films 114 and 116 cannot be clearly observed insome cases. Thus, in this embodiment, the boundary between theinsulating films 114 and 116 is shown by a dashed line. Although atwo-layer structure of the insulating films 114 and 116 is described inthis embodiment, the present invention is not limited to this. Forexample, a single-layer structure of the insulating film 114 may beused.

The insulating film 118 contains nitrogen. Alternatively, the insulatingfilm 118 contains nitrogen and silicon. The insulating film 118 has afunction of blocking oxygen, hydrogen, water, an alkali metal, analkaline earth metal, or the like. A nitride insulating film, forexample, can be used as the insulating film 118. The nitride insulatingfilm is formed using silicon nitride, silicon nitride oxide, aluminumnitride, aluminum nitride oxide, or the like. Note that instead of thenitride insulating film having a blocking effect against oxygen,hydrogen, water, an alkali metal, an alkaline earth metal, and the like,an oxide insulating film having a blocking effect against oxygen,hydrogen, water, and the like, may be provided. As the oxide insulatingfilm having a blocking effect against oxygen, hydrogen, water, and thelike, an aluminum oxide film, an aluminum oxynitride film, a galliumoxide film, a gallium oxynitride film, an yttrium oxide film, an yttriumoxynitride film, a hafnium oxide film, and a hafnium oxynitride film canbe given.

<Third Conductive Film>

The conductive film 120 contains indium and oxygen. Alternatively, theconductive film 120 contains indium, tin, and oxygen. Furtheralternatively, the conductive film 120 contains indium, tin, oxygen, andsilicon. The conductive film 120 can be formed using alight-transmitting conductive material such as indium tin oxide, indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium zinc oxide, or indium tin oxide towhich silicon oxide is added. Note that the conductive film 120 can beformed by a sputtering method or the like.

<Third Insulating Film>

The insulating film 122 can be formed using the material of theinsulating film 122 described above. As the insulating film 122, forexample, a silicon nitride film, a silicon nitride oxide film, a siliconoxide film, a silicon oxynitride film, an aluminum nitride film, analuminum nitride oxide film, an aluminum oxide film, or an aluminumoxynitride film formed with a PECVD apparatus can be used. An ammoniagas is not necessarily used as a deposition gas of the silicon nitridefilm which is formed with a PECVD apparatus. Without using an ammoniagas as a deposition gas, it is possible to reduce the amount of ammoniaentering the film. Therefore, the number of ammonia molecules releasedfrom the insulating film 122 can be made small.

<Structure Example 2 of Semiconductor Device>

Next, a structure example of a semiconductor device, which is differentfrom that of the semiconductor device described above, will be describedwith reference to FIGS. 2A and 2B. FIG. 2A is a top view of asemiconductor device of one embodiment of the present invention. FIG. 2Bis a cross-sectional view taken along a dashed dotted line A1-A2 in FIG.2A.

The semiconductor device illustrated in FIGS. 2A and 2B includes theconductive film 104 over the substrate 102, the insulating film 106 overthe substrate 102 and the conductive film 104, the conductive film 112over the insulating film 106, the insulating films 114, 116, and 118over the conductive film 112, the conductive film 120 electricallyconnected to the conductive film 104 through an opening 143 provided inthe insulating films 114, 116, and 118 and the opening 142 provided inthe insulating film 106, and the insulating film 122 over the conductivefilm 120.

The semiconductor device illustrated in FIGS. 2A and 2B is differentfrom the semiconductor device illustrated in FIGS. 1A and 1B in that theopening 143 is provided. As illustrated in FIGS. 2A and 2B, with astructure in which an end portion of the opening 143 provided in theinsulating films 114, 116, and 118 is provided outside an end portion ofthe opening 142 provided in the insulating film 106, coverage of theconductive film 120 and the insulating film 122 can be improved.

<Structure Example 3 of Semiconductor Device>

Next, a structure example of a semiconductor device, which is differentfrom those of the semiconductor devices described above, will bedescribed with reference to FIGS. 3A to 3C. FIG. 3A is a top view of atransistor 100 that is a semiconductor device of one embodiment of thepresent invention. FIG. 3B is a cross-sectional view taken along adashed dotted line X1-X2 in FIG. 3A, and FIG. 3C is a cross-sectionalview taken along a dashed dotted line Y1-Y2 in FIG. 3A. Furthermore, thedirection of the dashed dotted line X1-X2 may be called a channel lengthdirection, and the direction of the dashed dotted line Y1-Y2 may becalled a channel width direction.

The transistor 100 includes a conductive film 104 a functioning as agate electrode over the substrate 102, the insulating film 106 over thesubstrate 102 and the conductive film 104 a, an oxide semiconductor film108 over the insulating film 106, and conductive films 112 a and 112 bfunctioning as source and drain electrodes electrically connected to theoxide semiconductor film 108. Over the transistor 100, specifically,over the conductive films 112 a and 112 b and the oxide semiconductorfilm 108, the insulating films 114, 116, and 118 are provided. Anopening 142 a reaching the conductive film 112 b is provided in theinsulating films 114, 116, and 118, through which a conductive film 120a electrically connected to the conductive film 112 b is provided. Theinsulating film 122 is provided over the insulating film 118 and theconductive film 120 a. Note that the insulating film 122 is formed so asto cover the end portion of the conductive film 120 a, and theconductive film 120 a includes a region not covered with the insulatingfilm 122.

The insulating films 114, 116, and 118 collectively function as aprotective insulating film for the transistor 100. The insulating film122 functions as a protective insulating film for the transistor 100 anda protective insulating film for the conductive film 120 a. Theconductive film 120 a functions as a pixel electrode used for a displaydevice. The insulating film 106 functions as a gate insulating film ofthe transistor 100.

When oxygen vacancy is formed in the oxide semiconductor film 108included in the transistor 100, electrons serving as carriers aregenerated; as a result, the transistor 100 tends to have normally-oncharacteristics. Therefore, to obtain stable transistor characteristics,it is important to reduce oxygen vacancy in the oxide semiconductor film108. In the structure of the transistor of one embodiment of the presentinvention, excess oxygen is introduced into an insulating film over theoxide semiconductor film 108, here, the insulating film 114 over theoxide semiconductor film 108, whereby oxygen is moved from theinsulating film 114 to the oxide semiconductor film 108 to fill oxygenvacancy in the oxide semiconductor film 108. Alternatively, excessoxygen is introduced into the insulating film 116 over the oxidesemiconductor film 108, whereby oxygen is moved from the insulating film116 to the oxide semiconductor film 108 through the insulating film 114to fill oxygen vacancy in the oxide semiconductor film 108.Alternatively, excess oxygen is introduced into the insulating films 114and 116 over the oxide semiconductor film 108, whereby oxygen is movedfrom both the insulating films 114 and 116 to the oxide semiconductorfilm 108 to fill oxygen vacancy in the oxide semiconductor film 108.

Therefore, the insulating films 114 and 116 include oxygen. It ispreferable that the insulating films 114 and 116 each include a region(oxygen excess region) containing oxygen in excess of that in thestoichiometric composition. In other words, the insulating films 114 and116 are each an insulating film capable of releasing oxygen. Note thatthe oxygen excess region is formed in each of the insulating films 114and 116 in such a manner that oxygen is introduced into the insulatingfilms 114 and 116 after the deposition, for example. As a method forintroducing oxygen, an ion implantation method, an ion doping method, aplasma immersion ion implantation method, plasma treatment, or the likemay be employed.

In addition, the insulating film 122 functioning as a protectiveinsulating film, which is provided over the transistor 100, includes aregion from which a small number of ammonia molecules are released inthe TDS analysis described above. Therefore, moisture or ammoniaentering the oxide semiconductor film 108 of the transistor 100 can bereduced; thus, an impurity that might be bonded to an oxygen vacancy inthe oxide semiconductor film 108 (here, the impurity is hydrogen orammonia) is reduced. Accordingly, a highly reliable semiconductor devicecan be provided.

Next, other components of the transistor of this embodiment aredescribed below in detail. Note that as for the components similar tothose of the semiconductor device illustrated in FIGS. 1A and 1B andFIGS. 2A and 2B, description thereof is omitted here.

<Gate Electrode>

The conductive film 104 a functioning as a gate electrode of thetransistor 100 can be formed using a material and a method which aresimilar to those of the conductive film 104 described above.

<Gate Insulating Film>

The insulating film 106 functioning as a gate insulating film of thetransistor 100 can be formed using a material and a method which aresimilar to those of the insulating film 106 described above. Theinsulating film 106 functions as a blocking film which inhibitspenetration of oxygen. For example, in the case where excess oxygen issupplied to the insulating film 106 b, the insulating film 114, theinsulating film 116, and/or the oxide semiconductor film 108, theinsulating film 106 can inhibit penetration of oxygen.

Note that the insulating film 106 b that is in contact with the oxidesemiconductor film 108 functioning as a channel region of the transistor100 is preferably an oxide insulating film and further preferablyincludes a region containing oxygen in excess of the stoichiometriccomposition (oxygen-excess region). In other words, the insulating film106 b is an insulating film which is capable of releasing oxygen. Inorder to provide the oxygen excess region in the insulating film 106 b,the insulating film 106 b is formed in an oxygen atmosphere, forexample. Alternatively, the oxygen excess region may be formed byintroduction of oxygen into the insulating film 106 b after thedeposition. As a method for introducing oxygen, an ion implantationmethod, an ion doping method, a plasma immersion ion implantationmethod, plasma treatment, or the like may be employed.

In the case where hafnium oxide is used for the insulating film 106 b,the following effect is attained. Hafnium oxide has a higher dielectricconstant than silicon oxide and silicon oxynitride. Therefore, aphysical thickness can be made larger than an equivalent oxidethickness; thus, even in the case where the equivalent oxide thicknessis less than or equal to 10 nm or less than or equal to 5 nm, leakagecurrent due to tunnel current can be low. That is, it is possible toprovide a transistor with a low off-state current. Moreover, hafniumoxide with a crystalline structure has a higher dielectric constant thanhafnium oxide with an amorphous structure. Therefore, it is preferableto use hafnium oxide with a crystalline structure in order to provide atransistor with a low off-state current. Examples of the crystallinestructure include a monoclinic crystal structure and a cubic crystalstructure. Note that one embodiment of the present invention is notlimited thereto.

The silicon nitride film has a higher dielectric constant than a siliconoxide film and needs a larger thickness for capacitance equivalent tothat of the silicon oxide film. Thus, when the silicon nitride film isincluded in the gate insulating film of the transistor 100, the physicalthickness of the insulating film can be increased. This makes itpossible to reduce a decrease in withstand voltage of the transistor 100and furthermore to increase the withstand voltage, thereby reducingelectrostatic discharge damage to the transistor 100.

<Oxide Semiconductor Film>

The oxide semiconductor film 108 contains O, In, Zn, and M (M is Ti, Ga,Y, Zr, La, Ce, Nd, or Hf). Typically, In—Ga oxide, In—Zn oxide, orIn-M-Zn oxide can be used for the oxide semiconductor film 108. It isparticularly preferable to use In-M-Zn oxide for the oxide semiconductorfilm 108.

In the case where the oxide semiconductor film 108 is formed of In-M-Znoxide, it is preferable that the atomic ratio of metal elements of asputtering target used for forming the In-M-Zn oxide satisfy In≥M andZn≥M. As the atomic ratio of metal elements of such a sputtering target,In:M:Zn=1:1:1, InM:Zn=1:1:1.2, and In:M:Zn=3:1:2 are preferable. Notethat the atomic ratios of metal elements in the formed oxidesemiconductor film 108 vary from the above atomic ratio of metalelements of the sputtering target within a range of ±40% as an error.

Note that in the case where the oxide semiconductor film 108 is anIn-M-Zn oxide film, the proportion of In and the proportion of M, nottaking Zn and O into consideration, are preferably greater than or equalto 25 atomic % and less than 75 atomic %, respectively, or furtherpreferably greater than or equal to 34 atomic % and less than 66 atomic%, respectively.

The energy gap of the oxide semiconductor film 108 is 2 eV or more,preferably 2.5 eV or more, or further preferably 3 eV or more. With theuse of an oxide semiconductor having such a wide energy gap, theoff-state current of the transistor 100 can be reduced.

The thickness of the oxide semiconductor film 108 is greater than orequal to 3 nm and less than or equal to 200 nm, preferably greater thanor equal to 3 nm and less than or equal to 100 nm, or further preferablygreater than or equal to 3 nm and less than or equal to 50 nm.

An oxide semiconductor film with low carrier density is used as theoxide semiconductor film 108. For example, an oxide semiconductor filmwhose carrier density is lower than 8×10¹¹/cm³, preferably lower than1×10¹¹/cm³, further preferably lower than 1×10¹⁰/cm³, or still furtherpreferably lower than 1×10⁻⁹/cm³ is used as the oxide semiconductor film108.

Note that without limitation to the compositions and materials describedabove, a material with an appropriate composition may be used dependingon required semiconductor characteristics and electrical characteristics(e.g., field-effect mobility and threshold voltage) of a transistor.Furthermore, in order to obtain required semiconductor characteristicsof a transistor, it is preferable that the carrier density, the impurityconcentration, the defect density, the atomic ratio of a metal elementto oxygen, the interatomic distance, the density, and the like of theoxide semiconductor film 108 be set to be appropriate.

Note that it is preferable to use, as the oxide semiconductor film 108,an oxide semiconductor film in which the impurity concentration is lowand density of defect states is low, in which case the transistor canhave more excellent electrical characteristics. Here, the state in whichimpurity concentration is low and density of defect states is low (thenumber of oxygen vacancies is small) is referred to as “highly purifiedintrinsic” or “substantially highly purified intrinsic”. A highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor film has few carrier generation sources, and thus can havea low carrier density. Thus, a transistor in which a channel region isformed in the oxide semiconductor film rarely has a negative thresholdvoltage (is rarely normally on). A highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has alow density of defect states and accordingly has few carrier traps insome cases. Furthermore, the highly purified intrinsic or substantiallyhighly purified intrinsic oxide semiconductor film has an extremely lowoff-state current; even when an element has a channel width of 1×10⁶/μmand a channel length L of 10 μm, the off-state current can be less thanor equal to the measurement limit of a semiconductor parameter analyzer,i.e., less than or equal to 1×10⁻¹³ A, at a voltage (drain voltage)between a source electrode and a drain electrode of from 1 V to 10 V.

Accordingly, the transistor in which the channel region is formed in thehighly purified intrinsic or substantially highly purified intrinsicoxide semiconductor film can have a small variation in electricalcharacteristics and high reliability. Charges trapped by the trap statesin the oxide semiconductor film take a long time to be released and maybehave like fixed charges. Thus, the transistor whose channel region isformed in the oxide semiconductor film having a high density of trapstates has unstable electrical characteristics in some cases. Asexamples of the impurities, hydrogen, nitrogen, an alkali metal, analkaline earth metal, and the like are given.

Hydrogen included in the oxide semiconductor film reacts with oxygenbonded to a metal atom to be water, and also causes an oxygen vacancy ina lattice from which oxygen is released (or a portion from which oxygenis released). Due to entry of hydrogen into the oxygen vacancy, anelectron serving as a carrier is generated in some cases. Furthermore,in some cases, bonding of part of hydrogen to oxygen bonded to a metalelement causes generation of an electron serving as a carrier. Thus, atransistor including an oxide semiconductor film which contains hydrogenis likely to have normally-on characteristics. Accordingly, it ispreferable that hydrogen be reduced as much as possible in the oxidesemiconductor film 108. Specifically, in the oxide semiconductor film108, the concentration of hydrogen which is measured by secondary ionmass spectrometry (SIMS) is lower than or equal to 2×10²⁰ atoms/cm³,preferably lower than or equal to 5×10¹⁹ atoms/cm³, further preferablylower than or equal to 1×10¹⁹ atoms/cm³, further preferably lower thanor equal to 5×10¹⁸ atoms/cm³, further preferably lower than or equal to1×10¹⁸ atoms/cm³, further preferably lower than or equal to 5×10¹⁷atoms/cm³, or further preferably lower than or equal to 1×10¹⁶atoms/cm³.

When silicon or carbon that is one of elements belonging to Group 14 iscontained in the oxide semiconductor film 108, oxygen vacancy isincreased in the oxide semiconductor film 108, and the oxidesemiconductor film 108 becomes an n-type film. Thus, the concentrationof silicon or carbon (the concentration is measured by SIMS) in theoxide semiconductor film 108 or the concentration of silicon or carbon(the concentration is measured by SIMS) in the vicinity of an interfacewith the oxide semiconductor film 108 is set to be lower than or equalto 2×10¹⁸ atoms/cm³, or preferably lower than or equal to 2×10¹⁷atoms/cm³.

In addition, the concentration of an alkali metal or an alkaline earthmetal of the oxide semiconductor film 108, which is measured by SIMS, islower than or equal to 1×10¹⁸ atoms/cm³, or preferably lower than orequal to 2×10¹⁶ atoms/cm³. An alkali metal and an alkaline earth metalmight generate carriers when bonded to an oxide semiconductor, in whichcase the off-state current of the transistor might be increased.Therefore, it is preferable to reduce the concentration of an alkalimetal or an alkaline earth metal of the oxide semiconductor film 108.

Furthermore, when containing nitrogen, the oxide semiconductor film 108easily becomes n-type by generation of electrons serving as carriers andan increase of carrier density. Thus, a transistor including an oxidesemiconductor film which contains nitrogen is likely to have normally-oncharacteristics. For this reason, nitrogen in the oxide semiconductorfilm is preferably reduced as much as possible; the concentration ofnitrogen which is measured by SIMS is preferably set to be, for example,lower than or equal to 5×10¹⁸ atoms/cm³.

The oxide semiconductor film 108 may have a non-single-crystalstructure, for example. The non-single crystal structure includes ac-axis aligned crystalline oxide semiconductor (CAAC-OS) which isdescribed later, a polycrystalline structure, a microcrystallinestructure, or an amorphous structure, for example. Among the non-singlecrystal structure, the amorphous structure has the highest density ofdefect states, whereas the CAAC-OS has the lowest density of defectstates.

The oxide semiconductor film 108 may have a non-single-crystalstructure, for example. The oxide semiconductor films having theamorphous structure each have disordered atomic arrangement and nocrystalline component, for example. Alternatively, the oxide filmshaving an amorphous structure have, for example, an absolutely amorphousstructure and no crystal part.

Note that the oxide semiconductor film 108 may be a mixed film includingtwo or more of the following: a region having an amorphous structure, aregion having a microcrystalline structure, a region having apolycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure. The mixed film has a single-layer structureincluding, for example, two or more of a region having an amorphousstructure, a region having a microcrystalline structure, a region havinga polycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure in some cases. Furthermore, in some cases, themixed film has a stacked-layer structure of two or more of a regionhaving an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure.

<Protective Insulating Film>

The insulating films 114, 116, and 118 collectively function as aprotective insulating film. Note that the insulating film 114 alsofunctions as a film which relieves damage to the oxide semiconductorfilm 108 at the time of forming the insulating film 116 in a later step.

In addition, it is preferable that the number of defects in theinsulating film 114 be small and typically, the spin densitycorresponding to a signal that appears at g=2.001 due to a dangling bondof silicon be lower than or equal to 3×10¹⁷ spins/cm³ by electron spinresonance (ESR) measurement. This is because if the density of defectsin the insulating film 114 is high, oxygen is bonded to the defects andthe amount of oxygen that penetrates the insulating film 114 isdecreased.

Note that all oxygen entering the insulating film 114 from the outsidedoes not move to the outside of the insulating film 114 and some oxygenremains in the insulating film 114. Furthermore, movement of oxygenoccurs in the insulating film 114 in some cases in such a manner thatoxygen enters the insulating film 114 and oxygen included in theinsulating film 114 moves to the outside of the insulating film 114.When an oxide insulating film which is permeable to oxygen is formed asthe insulating film 114, oxygen released from the insulating film 116provided over the insulating film 114 can be moved to the oxidesemiconductor film 108 through the insulating film 114.

Note that the insulating film 114 can be formed using an oxideinsulating film having a low density of states due to nitrogen oxidebetween the energy level of the valence band maximum (E_(v_os)) and theenergy level of the conduction band minimum (E_(c_os)) of the oxidesemiconductor film. A silicon oxynitride film that releases lessnitrogen oxide, an aluminum oxynitride film that releases less nitrogenoxide, or the like can be used as the oxide insulating film in which thedensity of states due to nitrogen oxide is low between E_(v_os) andE_(c_os).

Note that a silicon oxynitride film that releases less nitrogen oxide isa film which releases more ammonia molecules than the nitrogen oxide inthermal desorption spectroscopy analysis; the number of ammoniamolecules released from the silicon oxynitride film is typically greaterthan or equal to 1×10¹⁸ molecules/cm³ and less than or equal to 5×10¹⁹molecules/cm³. Note that the number of ammonia molecules released from afilm is the number of ammonia molecules released by heat treatment withwhich the surface temperature of the film becomes higher than or equalto 50° C. and lower than or equal to 650° C., or preferably higher thanor equal to 50° C. and lower than or equal to 550° C.

Nitrogen oxide (NO_(x); x is greater than or equal to 0 and less than orequal to 2, or preferably greater than or equal to 1 and less than orequal to 2), typically NO₂ or NO, forms levels in the insulating film114, for example. The level is positioned in the energy gap of the oxidesemiconductor film 108. Therefore, when nitrogen oxide is diffused tothe interface between the insulating film 114 and the oxidesemiconductor film 108, an electron is trapped by the level on theinsulating film 114 side. As a result, the trapped electron remains inthe vicinity of the interface between the insulating film 114 and theoxide semiconductor film 108; thus, the threshold voltage of thetransistor is shifted in the positive direction.

Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Sincenitrogen oxide contained in the insulating film 114 reacts with ammoniacontained in the insulating film 116 in heat treatment, nitrogen oxidecontained in the insulating film 114 is reduced. Therefore, an electronis hardly trapped at the interface between the insulating film 114 andthe oxide semiconductor film 108.

By using, for the insulating film 114, the oxide insulating film havinga low density of states of nitrogen oxide between E_(v_os) and E_(c_os),the shift in the threshold voltage of the transistor can be reduced,which leads to a smaller change in the electrical characteristics of thetransistor.

Note that in an ESR spectrum at 100 K or lower of the insulating film114, by heat treatment of a manufacturing process of the transistor,typically heat treatment at a temperature higher than or equal to 300°C. and lower than the strain point of the substrate, a first signal thatappears at a g-factor greater than or equal to 2.037 and less than orequal to 2.039, a second signal that appears at a g-factor greater thanor equal to 2.001 and less than or equal to 2.003, and a third signalthat appears at a g-factor greater than or equal to 1.964 and less thanor equal to 1.966 are observed. The split width of the first and secondsignals and the split width of the second and third signals that areobtained by ESR measurement using an X-band are each approximately 5 mT.The sum of the spin densities of the first signal that appears at ag-factor greater than or equal to 2.037 and less than or equal to 2.039,the second signal that appears at a g-factor greater than or equal to2.001 and less than or equal to 2.003, and the third signal that appearsat a g-factor greater than or equal to 1.964 and less than or equal to1.966 is lower than 1×10¹⁸ spins/cm³, typically higher than or equal to1×10¹⁷ spins/cm³ and lower than 1×10¹⁸ spins/cm³.

In the ESR spectrum at 100 K or lower, the first signal that appears ata g-factor greater than or equal to 2.037 and less than or equal to2.039, the second signal that appears at a g-factor greater than orequal to 2.001 and less than or equal to 2.003, and the third signalthat appears at a g-factor greater than or equal to 1.964 and less thanor equal to 1.966 correspond to signals attributed to nitrogen oxide(NO_(x); x is greater than or equal to 0 and smaller than or equal to 2,or preferably greater than or equal to 1 and less than or equal to 2).Typical examples of nitrogen oxide include nitrogen monoxide andnitrogen dioxide. In other words, the lower the total spin density ofthe first signal that appears at a g-factor greater than or equal to2.037 and less than or equal to 2.039, the second signal that appears ata g-factor greater than or equal to 2.001 and less than or equal to2.003, and the third signal that appears at a g-factor greater than orequal to 1.964 and less than or equal to 1.966 is, the lower the contentof nitrogen oxide in the oxide insulating film is.

The concentration of nitrogen of the oxide insulating film having a lowdensity of states of nitrogen oxide between E_(v_os) and E_(c_os)measured by secondary mass spectrometry (SIMS) is lower than or equal to6×10²⁰ atoms/cm³.

The oxide insulating film in which the density of states of nitrogenoxide is low between E_(v_os) and E_(c_os) is formed by a PECVD methodat a substrate temperature higher than or equal to 220° C., higher thanor equal to 280° C., or higher than or equal to 350° C. using silane anddinitrogen monoxide, whereby a dense and hard film can be formed.

It is preferable that the number of defects in the insulating film 116be small, and typically the spin density corresponding to a signal thatappears at g=2.001 due to a dangling bond of silicon, be lower than1.5×10¹⁸ spins/cm³, or further preferably lower than or equal to 1×10¹⁸spins/cm³ by ESR measurement. Note that the insulating film 116 isprovided more apart from the oxide semiconductor film 108 than theinsulating film 114 is; thus, the insulating film 116 may have higherdensity of defects than the insulating film 114.

<Conductive Film Functioning as Pixel Electrode>

The conductive film 120 a can be formed using a material and a methodwhich are similar to those of the conductive film 120 described above.

Although the variety of films such as the conductive films, theinsulating films, and the oxide semiconductor films which are describedabove can be formed by a sputtering method or a PECVD method, such filmsmay be formed by another method, e.g., a thermal chemical vapordeposition (CVD) method or an atomic layer deposition (ALD) method. Asan example of a thermal CVD method, a metal organic chemical vapordeposition (MOCVD) method can be given.

A thermal CVD method has an advantage that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to the chamber at a timeso that the pressure in a chamber is set to an atmospheric pressure or areduced pressure, and react with each other in the vicinity of thesubstrate or over the substrate.

Deposition by an ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). For example, a first source gas is introduced, aninert gas (e.g., argon or nitrogen) or the like is introduced at thesame time as or after the introduction of the first gas so that thesource gases are not mixed, and then a second source gas is introduced.Note that in the case where the first source gas and the inert gas areintroduced at a time, the inert gas serves as a carrier gas, and theinert gas may also be introduced at the same time as the introduction ofthe second source gas. Alternatively, the first source gas may beexhausted by vacuum evacuation instead of the introduction of the inertgas, and then the second source gas may be introduced. The first sourcegas is adsorbed on the surface of the substrate to form a first layer;then the second source gas is introduced to react with the first layer;as a result, a second layer is stacked over the first layer, so that athin film is formed. The sequence of the gas introduction is repeatedplural times until a desired thickness is obtained, whereby a thin filmwith excellent step coverage can be formed. The thickness of the thinfilm can be adjusted by the number of repetition times of the sequenceof the gas introduction; therefore, an ALD method makes it possible toaccurately adjust a thickness and thus is suitable for manufacturing aminute FET.

The variety of films such as the conductive films, the insulating films,the oxide semiconductor films, and the metal oxide films in thisembodiment can be formed by a thermal CVD method such as an MOCVDmethod. For example, in the case where an In—Ga—Zn—O film is formed,trimethylindium, trimethylgallium, and dimethylzinc are used. Note thatthe chemical formula of trimethylindium is In(CH₃)₃. The chemicalformula of trimethylgallium is Ga(CH₃)₃. The chemical formula ofdimethylzinc is Zn(CH₃)₂. Without limitation to the above combination,triethylgallium (chemical formula: Ga(C₂H₅)₃) can be used instead oftrimethylgallium and diethylzinc (chemical formula: Zn(C₂H₅)₂) can beused instead of dimethylzinc.

For example, in the case where a hafnium oxide film is formed by adeposition apparatus using an ALD method, two kinds of gases, i.e.,ozone (O₃) as an oxidizer and a source gas which is obtained byvaporizing liquid containing a solvent and a hafnium precursor compound(e.g., a hafnium alkoxide or a hafnium amide such astetrakis(dimethylamide)hafnium (TDMAH)) are used. Note that the chemicalformula of tetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄. Examples ofanother material liquid include tetrakis(ethylmethylamide)hafnium.

For example, in the case where an aluminum oxide film is formed by adeposition apparatus using an ALD method, two kinds of gases, e.g., H₂Oas an oxidizer and a source gas which is obtained by vaporizing liquidcontaining a solvent and an aluminum precursor compound (e.g.,trimethylaluminum (TMA)) are used. Note that the chemical formula oftrimethylaluminum is Al(CH₃)₃. Examples of another material liquidinclude tris(dimethylamide)aluminum, triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed by adeposition apparatus using an ALD method, hexachlorodisilane is adsorbedon a surface where a film is to be formed, chlorine included in theadsorbate is removed, and radicals of an oxidizing gas (e.g., O₂ ordinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is formed by a depositionapparatus using an ALD method, a WF₆ gas and a B₂H₆ gas are sequentiallyintroduced plural times to form an initial tungsten film, and then a WF₆gas and an H₂ gas are sequentially introduced plural times to form atungsten film. Note that an SiH₄ gas may be used instead of a B₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., anIn—Ga—Zn—O film is formed by a deposition apparatus using an ALD method,an In(CH₃)₃ gas and an O₃ gas are sequentially introduced plural timesto form an In—O layer, a Ga(CH₃)₃ gas and an O₃ gas are sequentiallyintroduced plural times to form a GaO layer, and then a Zn(CH₃)₂ gas andan O₃ gas are sequentially introduced plural times to form a ZnO layer.Note that the order of these layers is not limited to this example. Amixed compound layer such as an In—Ga—O layer, an In—Zn—O layer, or aGa—Zn—O layer may be formed by using of these gases. Note that althoughan H₂O gas which is obtained by bubbling with an inert gas such as Armay be used instead of an O₃ gas, it is preferable to use an O₃ gas,which does not contain H. Furthermore, instead of an In(CH₃)₃ gas, anIn(C₂H₅)₃ gas may be used. Instead of a Ga(CH₃)₃ gas, a Ga(C₂H₅)₃ gasmay be used. Furthermore, a Zn(CH₃)₂ gas may be used.

<Structure Example 4 of Semiconductor Device>

Next, a structure example of a semiconductor device, which is differentfrom those of the semiconductor devices described above, will bedescribed with reference to FIGS. 4A to 4C. FIG. 4A is a top view of atransistor 150 that is a semiconductor device of one embodiment of thepresent invention. FIG. 4B is a cross-sectional view taken along adashed dotted line X1-X2 in FIG. 4A, and FIG. 4C is a cross-sectionalview taken along a dashed dotted line Y1-Y2 in FIG. 4A.

The transistor 150 includes the conductive film 104 a over the substrate102, the insulating film 106 over the substrate 102 and the conductivefilm 104 a, the oxide semiconductor film 108 over the insulating film106, the insulating film 114 over the oxide semiconductor film 108, theinsulating film 116 over the insulating film 114, and the conductivefilms 112 a and 112 b functioning as source and drain electrodeselectrically connected to the oxide semiconductor film 108 thoughopenings 141 a and 141 b provided in the insulating films 114 and 116.Over the transistor 150, specifically, over the conductive films 112 aand 112 b and the insulating film 116, the insulating film 118 isprovided. An opening 142 b reaching the conductive film 112 b isprovided in the insulating film 118, through which the conductive film120 a electrically connected to the conductive film 112 b is provided.The insulating film 122 is provided over the insulating film 118 and theconductive film 120 a. Note that the insulating film 122 is formed so asto cover the end portion of the conductive film 120 a, and theconductive film 120 a includes a region not covered with the insulatingfilm 122.

The insulating films 114 and 116 collectively function as a protectiveinsulating film for the oxide semiconductor film 108. The insulatingfilm 118 functions as a protective insulating film for the transistor150. The insulating film 122 functions as a protective insulating filmfor the transistor 150 and a protective insulating film for theconductive film 120 a. The conductive film 120 a functions as a pixelelectrode used for a display device. The insulating film 106 functionsas a gate insulating film of the transistor 150.

Although the transistor 100 described above has a channel-etchedstructure, the transistor 150 in FIGS. 4A to 4C has a channel-protectivestructure. Thus, either the channel-etched structure or thechannel-protective structure can be applied to the semiconductor deviceof one embodiment of the present invention.

Like the transistor 100, the transistor 150 is provided with theinsulating film 114 over the oxide semiconductor film 108; therefore,oxygen contained in the insulating film 114 or oxygen contained in theinsulating film 116 can fill an oxygen vacancy in the oxidesemiconductor film 108. In addition, the insulating film 122 functioningas a protective insulating film is provided over the transistor 150;therefore, impurities that might be bonded to oxygen vacancies in theoxide semiconductor film 108 are reduced.

Since the insulating film 122 is provided over the transistor 150, entryof moisture from the outside can be suppressed.

<Structure Example 5 of Semiconductor Device>

Next, a structure example of a semiconductor device, which is differentfrom those of the semiconductor devices described above, will bedescribed with reference to FIGS. 5A to 5C. FIG. 5A is a top view of atransistor 160 that is a semiconductor device of one embodiment of thepresent invention. FIG. 5B is a cross-sectional view taken along adashed dotted line X1-X2 in FIG. 5A, and FIG. 5C is a cross-sectionalview taken along a dashed dotted line Y1-Y2 in FIG. 5A.

The transistor 160 includes the conductive film 104 a over the substrate102, the insulating film 106 over the substrate 102 and the conductivefilm 104 a, the oxide semiconductor film 108 over the insulating film106, the insulating film 114 over the oxide semiconductor film 108, theinsulating film 116 over the insulating film 114, and the conductivefilms 112 a and 112 b functioning as source and drain electrodeselectrically connected to the oxide semiconductor film 108. Over thetransistor 160, specifically, over the conductive films 112 a and 112 band the insulating film 116, the insulating film 118 is provided. Theopening 142 b reaching the conductive film 112 b is provided in theinsulating film 118, through which the conductive film 120 aelectrically connected to the conductive film 112 b is provided. Theinsulating film 122 is provided over the insulating film 118 and theconductive film 120 a. Note that the insulating film 122 is formed so asto cover the end portion of the conductive film 120 a, and theconductive film 120 a includes a region not covered with the insulatingfilm 122.

The insulating films 114 and 116 collectively function as a protectiveinsulating film for the oxide semiconductor film 108. The insulatingfilm 118 functions as a protective insulating film for the transistor160. The insulating film 122 functions as a protective insulating filmfor the transistor 160 and a protective insulating film for theconductive film 120 a. The conductive film 120 a functions as a pixelelectrode used for a display device. The insulating film 106 functionsas a gate insulating film of the transistor 160.

The transistor 160 is different from the transistor 150 described abovein the shapes of the insulating films 114 and 116. Specifically, theinsulating films 114 and 116 of the transistor 160 have an island shapeand are provided over a channel region of the oxide semiconductor film108. The other components are the same as those of the transistor 150,and the effect similar to that in the case of the transistor 150 isobtained.

<Structure Example 6 of Semiconductor Device>

Next, a structure example of a semiconductor device, which is differentfrom those of the semiconductor devices described above, will bedescribed with reference to FIGS. 6A to 6C. FIG. 6A is a top view of atransistor 170 that is a semiconductor device of one embodiment of thepresent invention. FIG. 6B is a cross-sectional view taken along adashed dotted line X1-X2 in FIG. 6A, and FIG. 6C is a cross-sectionalview taken along a dashed dotted line Y1-Y2 in FIG. 6A.

The transistor 170 includes the conductive film 104 a over the substrate102, the insulating film 106 over the substrate 102 and the conductivefilm 104 a, the oxide semiconductor film 108 over the insulating film106, the insulating film 114 over the oxide semiconductor film 108, theinsulating film 116 over the insulating film 114, and the conductivefilms 112 a and 112 b functioning as source and drain electrodeselectrically connected to the oxide semiconductor film 108. Over thetransistor 170, specifically, over the conductive films 112 a and 112 band the insulating film 116, the insulating film 118 is provided. Theopening 142 a reaching the conductive film 112 b is provided in theinsulating films 114, 116, and 118, through which the conductive film120 a electrically connected to the conductive film 112 b is provided.The conductive film 120 b is formed over the insulating film 118 tooverlap with the oxide semiconductor film 108. The insulating film 122is provided over the insulating film 118 and the conductive films 120 aand 120 b. Note that the insulating film 122 is formed so as to coverthe end portion of the conductive film 120 a, and the conductive film120 a includes a region not covered with the insulating film 122.

The insulating films 114 and 116 collectively function as a protectiveinsulating film for the oxide semiconductor film 108. The insulatingfilm 118 functions as a protective insulating film for the transistor170. The insulating film 122 functions as a protective insulating filmfor the transistor 170 and a protective insulating film for theconductive films 120 a and 120 b. The conductive film 120 a functions asa pixel electrode used for a display device. The insulating film 106functions as a gate insulating film of the transistor 170.

The conductive film 104 a in the transistor 170 functions as a firstgate electrode. The insulating film 106 in the transistor 170 functionsas gate insulating film. The insulating films 114, 116, and 118collectively function as a second gate insulating film of the transistor170. The conductive film 120 b in the transistor 170 functions as asecond gate electrode (also referred to as a back gate electrode).

As illustrated in FIG. 6C, the conductive film 120 b is connected to theconductive film 104 a functioning as a first gate electrode throughopenings 142 c and 142 d provided in the insulating films 106, 114, 116,and 118. Accordingly, the conductive film 120 b and the conductive film104 a are supplied with the same potential.

Note that although the structure in which the openings 142 c and 142 dare provided so that the conductive film 120 b and the conductive film104 a are connected to each other is described in this embodiment, oneembodiment of the present invention is not limited thereto. For example,a structure in which only one of the openings 142 c and 142 d isprovided so that the conductive film 120 b and the conductive film 104 aare connected to each other, or a structure in which the openings 142 cand 142 d are not provided and the conductive film 120 b and theconductive film 104 a are not connected to each other may be employed.Note that in the case where the conductive film 120 b and the conductivefilm 104 a are not connected to each other, it is possible to applydifferent potentials to the conductive film 120 b and the conductivefilm 104 a.

As illustrated in FIG. 6B, the oxide semiconductor film 108 ispositioned to face each of the conductive film 104 a functioning as afirst gate electrode and the conductive film 120 b functioning as asecond gate electrode, and is sandwiched between the two conductivefilms functioning as gate electrodes. The lengths in the channel lengthdirection and the channel width direction of the conductive film 120 bfunctioning as a second gate electrode are longer than those in thechannel length direction and the channel width direction of the oxidesemiconductor film 108. The whole oxide semiconductor film 108 iscovered with the conductive film 120 b with the insulating films 114,116, and 118 positioned therebetween. Since the conductive film 120 bfunctioning as a second gate electrode is connected to the conductivefilm 104 a functioning as a first gate electrode through the opening 142c and 142 d provided in the insulating films 106, 114, 116, and 118, aside surface of the oxide semiconductor film 108 in the channel widthdirection faces the conductive film 120 b functioning as a second gateelectrode with the insulating films 114, 116, and 118 positionedtherebetween.

In other words, in the channel width direction of the transistor 170,the conductive film 104 a functioning as a first gate electrode and theconductive film 120 b functioning as a second gate electrode areconnected to each other through the openings provided in the insulatingfilm 106 functioning as a first gate insulating film, and the insulatingfilms 114, 116, and 118 collectively functioning as a second gateinsulating film; and the conductive film 104 a and the conductive film120 b surround the oxide semiconductor film 108 with the insulating film106 functioning as a first gate insulating film, and the insulatingfilms 114, 116, and 118 collectively functioning as a second gateinsulating film positioned therebetween.

Such a structure makes it possible that the oxide semiconductor film 108included in the transistor 170 is electrically surrounded by electricfields of the conductive film 104 a functioning as a first gateelectrode and the conductive film 120 b functioning as a second gateelectrode. A device structure of a transistor, like that of thetransistor 170, in which electric fields of a first gate electrode and asecond gate electrode electrically surround an oxide semiconductor filmwhere a channel region is formed can be referred to as a surroundedchannel (s-channel) structure.

Since the transistor 170 has the s-channel structure, an electric fieldfor inducing a channel can be effectively applied to the oxidesemiconductor film 108 by the conductive film 104 a functioning as afirst gate electrode; therefore, the current drive capability of thetransistor 170 can be improved and high on-state current characteristicscan be obtained. Since the on-state current can be increased, it ispossible to reduce the size of the transistor 170. In addition, sincethe transistor 170 is surrounded by the conductive film 104 afunctioning as a first gate electrode and the conductive film 120 bfunctioning as a second gate electrode, the mechanical strength of thetransistor 170 can be increased.

<Structure Example 7 of Semiconductor Device>

Next, a structure example of a semiconductor device, which is differentfrom those of the semiconductor devices described above, will bedescribed with reference to FIGS. 7A to 7D. FIGS. 7A and 7B eachillustrate a cross-sectional view of a modification example of thetransistor 100 in FIGS. 3B and 3C. FIGS. 7C and 7D each illustrate across-sectional view of another modification example of the transistor100 in FIGS. 3B and 3C. Note that top views of the transistorsillustrated in FIGS. 7A to 7D are omitted here because they are similarto the top view of FIG. 3A.

A transistor 100A illustrated in FIGS. 7A and 7B has the same structureas the transistor 100 illustrated in FIGS. 3B and 3C except that theoxide semiconductor film 108 has a three-layer structure. Specifically,the oxide semiconductor film 108 of the transistor 100A includes anoxide semiconductor film 108 a, an oxide semiconductor film 108 b, andan oxide semiconductor film 108 c.

A transistor 100B illustrated in FIGS. 7C and 7D has the same structureas the transistor 100 in FIGS. 3B and 3C except that the oxidesemiconductor film 108 has a two-layer structure. Specifically, theoxide semiconductor film 108 of the transistor 100B includes the oxidesemiconductor film 108 a and the oxide semiconductor film 108 b.

Here, a band structure including the oxide semiconductor films 108 a,108 b, and 108 c and insulating films in contact with the oxidesemiconductor film 108 is described with reference to FIGS. 8A and 8B.

FIG. 8A shows an example of a band structure in the thickness directionof a stack of the insulating film 106 b, the oxide semiconductor films108 a, 108 b, and 108 c, and the insulating film 114. FIG. 8B shows anexample of a band structure in the thickness direction of a stack of theinsulating film 106 b, the oxide semiconductor films 108 a and 108 b,and the insulating film 114. For easy understanding, the energy level ofthe conduction band minimum (Ec) of each of the insulating film 106 b,the oxide semiconductor films 108 a, 108 b, and 108 c, and theinsulating film 114 is shown in the band structures.

In FIG. 8A, a silicon oxide film is used as each of the insulating films106 b and 114, an oxide semiconductor film formed using a metal oxidetarget having an atomic ratio of metal elements of In:Ga:Zn=1:1:1 isused as the oxide semiconductor film 108 a, an oxide semiconductor filmformed using a metal oxide target having an atomic ratio of metalelements of In:Ga:Zn=1:4:5 is used as the oxide semiconductor film 108b, and an oxide semiconductor film formed using a metal oxide targethaving an atomic ratio of metal elements of In:Ga:Zn=1:3:6 is used asthe oxide semiconductor film 108 c.

In the band structure of FIG. 8B, a silicon oxide film is used as eachof the insulating films 106 b and 114, an oxide semiconductor filmformed using a metal oxide target having an atomic ratio of metalelements of In:Ga:Zn=1:1:1 is used as the oxide semiconductor film 108a, and a metal oxide film formed using a metal oxide target having anatomic ratio of metal elements of In:Ga:Zn=1:3:6 is used as the oxidesemiconductor film 108 b.

As illustrated in FIGS. 8A and 8B, the energy level of the conductionband minimum smoothly varies between the oxide semiconductor film 108 aand the oxide semiconductor film 108 b. In other words, the conductionband minimum is continuously varied or a continuous junction is formed.To obtain such a band structure, there exist no impurity, which forms adefect state such as a trap center or a recombination center for theoxide semiconductor, at the interface between the oxide semiconductorfilm 108 a and the oxide semiconductor film 108 b.

To form a continuous junction between the oxide semiconductor film 108 aand the oxide semiconductor film 108 b, it is necessary to form thefilms successively without exposure to the air by using a multi-chamberdeposition apparatus (sputtering apparatus) provided with a load lockchamber.

With the band structure of FIG. 8A or FIG. 8B, the oxide semiconductorfilm 108 a serves as a well, and a channel region is formed in the oxidesemiconductor film 108 a in the transistor with the above stacked-layerstructure.

By providing the oxide semiconductor film 108 b and/or the oxidesemiconductor film 108 c, the oxide semiconductor film 108 a can bedistanced away from trap states.

In addition, the trap states might be more distant from the vacuum levelthan the energy level of the conduction band minimum (Ec) of the oxidesemiconductor film 108 a functioning as a channel region, so thatelectrons are likely to be accumulated in the trap states. When theelectrons are accumulated in the trap states, the electrons becomenegative fixed electric charge, so that the threshold voltage of thetransistor is shifted in the positive direction. Therefore, it ispreferable that the trap states be closer to the vacuum level than theenergy level of the conduction band minimum (Ec) of the oxidesemiconductor film 108 a. Such a structure inhibits accumulation ofelectrons in the trap states. As a result, the on-state current and thefield-effect mobility of the transistor can be increased.

In FIGS. 8A and 8B, the energy level of the conduction band minimum ofeach of the oxide semiconductor films 108 b and 108 c is closer to thevacuum level than that of the oxide semiconductor film 108 a. Typically,an energy difference between the conduction band minimum of the oxidesemiconductor film 108 a and the conduction band minimum of each of theoxide semiconductor films 108 b and 108 c is greater than or equal to0.15 eV or greater than or equal to 0.5 eV, and less than or equal to 2eV or less than or equal to 1 eV. That is, the difference between theelectron affinity of each of the oxide semiconductor films 108 b and 108c and the electron affinity of the oxide semiconductor film 108 a isgreater than or equal to 0.15 eV or greater than or equal to 0.5 eV, andless than or equal to 2 eV or less than or equal to 1 eV.

In such a structure, the oxide semiconductor film 108 a serves as a mainpath of current and functions as a channel region. In addition, sincethe oxide semiconductor films 108 b and 108 c each include one or moremetal elements included in the oxide semiconductor film 108 a in which achannel region is formed, interface scattering is less likely to occurat the interface between the oxide semiconductor film 108 a and theoxide semiconductor film 108 b. Thus, the transistor can have highfield-effect mobility because the movement of carriers is not hinderedat the interface.

To prevent each of the oxide semiconductor films 108 b and 108 c fromfunctioning as part of a channel region, a material having sufficientlylow conductivity is used for the oxide semiconductor films 108 b and 108c. Alternatively, a material which has a smaller electron affinity (adifference in energy level between the vacuum level and the energy levelof the conduction band minimum) than the oxide semiconductor film 108 aand has a difference in the energy level of the conduction band minimumfrom the oxide semiconductor film 108 a (band offset) is used for theoxide semiconductor films 108 b and 108 c. Furthermore, to inhibitgeneration of a difference between threshold voltages due to the valueof the drain voltage, it is preferable to form the oxide semiconductorfilms 108 b and 108 c using a material whose energy level of theconduction band minimum is closer to the vacuum level than that of theoxide semiconductor film 108 a by 0.2 eV or more, preferably 0.5 eV ormore.

It is preferable that the oxide semiconductor films 108 b and 108 c nothave a spinel crystal structure. This is because if the oxidesemiconductor films 108 b and 108 c have a spinel crystal structure,components of the conductive films 112 a and 112 b might be diffusedinto the oxide semiconductor film 108 a at the interface between thespinel crystal structure and another region. Note that each of the oxidesemiconductor films 108 b and 108 c is preferably a CAAC-OS, which isdescribed later, in which case a higher blocking property againstcomponents of the conductive films 112 a and 112 b, e.g., copperelements, is obtained.

The thickness of each of the oxide semiconductor films 108 b and 108 cis greater than or equal to a thickness that is capable of inhibitingdiffusion of the components of the conductive films 112 a and 112 b intothe oxide semiconductor film 108 a, and less than a thickness thatinhibits supply of oxygen from the insulating film 114 to the oxidesemiconductor film 108 a. For example, when the thickness of each of theoxide semiconductor films 108 b and 108 c is greater than or equal to 10nm, the components of the conductive films 112 a and 112 b can beprevented from diffusing into the oxide semiconductor film 108 a. Whenthe thickness of each of the oxide semiconductor films 108 b and 108 cis less than or equal to 100 nm, oxygen can be effectively supplied fromthe insulating films 114 and 116 to the oxide semiconductor film 108 a.

When the oxide semiconductor films 108 b and 108 c are each an In-M-Znoxide in which the atomic ratio of the element M (M is Ti, Ga, Y, Zr,La, Ce, Nd, Sn, or Hf) is higher than that of In, the energy gap of eachof the oxide semiconductor films 108 b and 108 c can be large and theelectron affinity thereof can be small. Therefore, a difference inelectron affinity between the oxide semiconductor film 108 a and each ofthe oxide semiconductor films 108 b and 108 c may be controlled by theproportion of the element M. Furthermore, an oxygen vacancy is lesslikely to be generated in the oxide semiconductor film in which theatomic ratio of Ti, Ga, Y, Zr, La, Ce, Nd, Sn, or Hf is higher than thatof In because Ti, Ga, Y, Zr, La, Ce, Nd, Sn, and Hf each are a metalelement that is strongly bonded to oxygen.

When an In-M-Zn oxide is used for the oxide semiconductor films 108 band 108 c, the proportions of In and M, not taking Zn and O intoconsideration, is preferably as follows: the atomic percentage of In isless than 50 atomic % and the atomic percentage of M is greater than orequal to 50 atomic %; further preferably, the atomic percentage of In isless than 25 atomic % and the atomic percentage of M is greater than orequal to 75 atomic %. Alternatively, a gallium oxide film may be used aseach of the oxide semiconductor films 108 b and 108 c.

Furthermore, in the case where each of the oxide semiconductor films 108a, 108 b, and 108 c is an In-M-Zn oxide, the proportion of M atoms ineach of the oxide semiconductor films 108 b and 108 c is higher thanthat in the oxide semiconductor film 108 a. Typically, the proportion ofM in each of the oxide semiconductor films 108 b and 108 c is 1.5 ormore times, preferably twice or more, or further preferably three ormore times as high as that in the oxide semiconductor film 108 a.

Furthermore, in the case where the oxide semiconductor films 108 a, 108b, and 108 c are each an In-M-Zn oxide, when the oxide semiconductorfilm 108 a has an atomic ratio of In:M:Zn=x₁:y₁:z₁ and the oxidesemiconductor films 108 b and 108 c each have an atomic ratio ofIn:M:Zn=x₂:y₂:z₂, y₂/x₂ is larger than y₁/x₁, preferably y₂/x₂ is 1.5 ormore times as large as y₁/x₁, further preferably, y₂/x₂ is two or moretimes as large as y₁/x₁, or still further preferably y₂/x₂ is three ormore times or four or more times as large as y₁/x₁. At this time, y₁ ispreferably greater than or equal to x₁ in the oxide semiconductor film108 a, because stable electrical characteristics of the transistor canbe achieved. However, when y₁ is three or more times as large as x₁, thefield-effect mobility of the transistor including the oxidesemiconductor film 108 a is reduced. Accordingly, y₁ is preferablysmaller than three times x₁.

In the case where the oxide semiconductor film 108 a is an In-M-Zn oxideand a target having the atomic ratio of metal elements ofIn:M:Zn=x₁:y₁:z₁ is used for depositing the oxide semiconductor film 108a, x₁/y₁ is preferably greater than or equal to ⅓ and less than or equalto 6, further preferably greater than or equal to 1 and less than orequal to 6, and z₁/y₁ is preferably greater than or equal to ⅓ and lessthan or equal to 6, further preferably greater than or equal to 1 andless than or equal to 6. Note that when z₁/y₁ is greater than or equalto 1 and less than or equal to 6, a CAAC-OS to be described later iseasily formed as the oxide semiconductor film 108 a. Typical examples ofthe atomic ratio of the metal elements of the target are In:M:Zn=1:1:1and In:M:Zn=3:1:2.

In the case where the oxide semiconductor films 108 b and 108 c are eachan In-M-Zn oxide and a target having an atomic ratio of metal elementsof InM:Zn=x₂:y₂:z₂ is used for depositing the oxide semiconductor films108 b and 108 c, x₂/y₂ is preferably less than x₁/y₁, and z₂/y₂ ispreferably greater than or equal to ⅓ and less than or equal to 6,further preferably greater than or equal to 1 and less than or equal to6. When the atomic ratio of M with respect to indium is high, the energygap of the oxide semiconductor films 108 b and 108 c can be large andthe electron affinity thereof can be small; therefore, y₂/x₂ ispreferably higher than or equal to 3 or higher than or equal to 4.Typical examples of the atomic ratio of the metal elements of the targetinclude In:M:Zn=1:3:2, In:M:Zn=1:3:4, In:M:Zn=1:3:5, In:M:Zn=1:3:6,In:M:Zn=1:4:2, In:M:Zn=1:4:4, In:M:Zn=1:4:5, and In:M:Zn=1:5:5.

Furthermore, in the case where the oxide semiconductor films 108 b and108 c are each an In-M oxide, when a divalent metal element (e.g., zinc)is not included as M, the oxide semiconductor films 108 b and 108 cwhich do not include a spinel crystal structure can be formed. As theoxide semiconductor films 108 b and 108 c, for example, an In—Ga oxidefilm can be used. The In—Ga oxide can be formed by a sputtering methodusing an In—Ga metal oxide target (In:Ga=7:93), for example. To depositthe oxide semiconductor films 108 b and 108 c by a sputtering methodusing DC discharge, on the assumption that an atomic ratio of In:M isx:y, it is preferable that y/(x+y) be less than or equal to 0.96, orfurther preferably less than or equal to 0.95, for example, 0.93.

In each of the oxide semiconductor films 108 a, 108 b, and 108 c, theproportions of the atoms in the above atomic ratio vary within a rangeof ±40% as an error.

The structures of the transistors of this embodiment can be freelycombined with each other.

<Method 1 for Manufacturing Semiconductor Device>

Next, a method for manufacturing a semiconductor device of oneembodiment of the present invention is described below in detail withreference to FIGS. 9A to 9C, FIGS. 10A and 10B, FIGS. 11A and 11B, andFIGS. 12A and 12B.

Note that as the semiconductor device of one embodiment of the presentinvention, the semiconductor device in FIGS. 1A and 1B and thetransistor 100 in FIGS. 3A to 3C can be formed in the same process.Therefore, the manufacturing method in FIGS. 9A to 9C, FIGS. 10A and10B, FIGS. 11A and 11B, and FIGS. 12A and 12B illustrates both amanufacturing method of the semiconductor device in FIGS. 1A and 1B andthat of the transistor 100 in FIGS. 3A to 3C.

Note that the films included in the semiconductor device (i.e., theinsulating film, the oxide semiconductor film, the conductive film, andthe like) can be formed by any of a sputtering method, a chemical vapordeposition (CVD) method, a vacuum evaporation method, and a pulsed laserdeposition (PLD) method. Alternatively, a coating method or a printingmethod can be used. Although the sputtering method and a PECVD methodare typical examples of the film formation method, a thermal CVD methodmay be used. As the thermal CVD method, a metal organic chemical vapordeposition (MOCVD) method or an atomic layer deposition (ALD) method maybe used, for example.

Deposition by the thermal CVD method may be performed in such a mannerthat the pressure in a chamber is set to an atmospheric pressure or areduced pressure, and a source gas and an oxidizer are supplied to thechamber at a time and react with each other in the vicinity of thesubstrate or over the substrate. Thus, no plasma is generated in thedeposition; therefore, the thermal CVD method has an advantage that nodefect due to plasma damage is caused.

Deposition by the ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching switching valves (also referred to ashigh-speed valves). In such a case, a first source gas is introduced, aninert gas (e.g., argon or nitrogen) or the like is introduced at thesame time as or after introduction of the first gas so that the sourcegases are not mixed, and then a second source gas is introduced. Notethat in the case where the first source gas and the inert gas areintroduced at a time, the inert gas serves as a carrier gas, and theinert gas may also be introduced at the same time as the introduction ofthe second source gas. Alternatively, the first source gas may beexhausted by vacuum evacuation instead of the introduction of the inertgas, and then the second source gas may be introduced. The first sourcegas is adsorbed on the surface of the substrate to form a firstsingle-atomic layer; then the second source gas is introduced to reactwith the first single-atomic layer; as a result, a second single-atomiclayer is stacked over the first single-atomic layer, so that a thin filmis formed.

The sequence of the gas introduction is repeated plural times until adesired thickness is obtained, whereby a thin film with excellent stepcoverage can be formed. The thickness of the thin film can be adjustedby the number of repetition times of the sequence of the gasintroduction; therefore, an ALD method makes it possible to accuratelyadjust a thickness and thus is suitable for manufacturing a minutetransistor.

First, a conductive film is formed over the substrate 102 and processedthrough a lithography process and an etching process, whereby theconductive film 104 and the conductive film 104 a functioning as a gateelectrode of the transistor 100 are formed. Then, the insulating films106 a and 106 b are formed over the conductive films 104 and 104 a (seeFIG. 9A).

The conductive film 104 and the conductive film 104 a functioning as agate electrode can be formed by a sputtering method, a CVD method, avacuum evaporation method, or a PLD method. Alternatively, a coatingmethod or a printing method can be used. Although typical depositionmethods are a sputtering method and PECVD method, a thermal CVD method,such as an MOCVD method, or an ALD method described above may be used.

In this embodiment, a glass substrate is used as the substrate 102, andas the conductive film 104 and the conductive film 104 a functioning asa gate electrode, a 100-nm-thick tungsten film is formed by a sputteringmethod.

The insulating films 106 a and 106 b can be formed by a sputteringmethod, a PECVD method, a thermal CVD method, a vacuum evaporationmethod, a PLD method, or the like. In this embodiment, a 400-nm-thicksilicon nitride film as the insulating film 106 a and a 50-nm-thicksilicon oxynitride film as the insulating film 106 b are formed by aPECVD method.

Note that the insulating film 106 a can have a stacked-layer structureof silicon nitride films. Specifically, the insulating film 106 a canhave a three-layer structure of a first silicon nitride film, a secondsilicon nitride film, and a third silicon nitride film. An example ofthe three-layer structure is as follows.

For example, the first silicon nitride film can be formed to have athickness of 50 nm under the condition where silane at a flow rate of200 sccm, nitrogen at a flow rate of 2000 sccm, and an ammonia gas at aflow rate of 100 sccm are supplied as a source gas to a reaction chamberof a PECVD apparatus; the pressure in the reaction chamber is controlledto 100 Pa, and a power of 2000 W is supplied using a 27.12 MHzhigh-frequency power source.

The second silicon nitride film can be formed to have a thickness of 300nm under the condition where silane at a flow rate of 200 sccm, nitrogenat a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 2000sccm are supplied as a source gas to the reaction chamber of the PECVDapparatus; the pressure in the reaction chamber is controlled to 100 Pa,and a power of 2000 W is supplied using a 27.12 MHz high-frequency powersource.

The third silicon nitride film can be formed to have a thickness of 50nm under the condition where silane at a flow rate of 200 sccm andnitrogen at a flow rate of 5000 sccm are supplied as a source gas to thereaction chamber of the PECVD apparatus; the pressure in the reactionchamber is controlled to 100 Pa, and a power of 2000 W is supplied usinga 27.12 MHz high-frequency power source.

Note that the first silicon nitride film, the second silicon nitridefilm, and the third silicon nitride film can be each formed at asubstrate temperature of 350° C.

When the insulating film 106 a has the three-layer structure of siliconnitride films, for example, in the case where a conductive filmincluding Cu is used as the conductive films 104 and 104 a, thefollowing effect can be obtained.

The first silicon nitride film can inhibit diffusion of a copper (Cu)element from the conductive films 104 and 104 a. The second siliconnitride film has a function of releasing hydrogen and can improvewithstand voltage of the insulating film functioning as a gateinsulating film. The third silicon nitride film releases a small amountof hydrogen and can inhibit diffusion of hydrogen released from thesecond silicon nitride film.

The insulating film 106 b is preferably an insulating film containingoxygen to improve characteristics of an interface with the oxidesemiconductor film 108 formed later.

Next, the oxide semiconductor film 108 is formed over the insulatingfilm 106 b (see FIG. 9B).

In this embodiment, an oxide semiconductor film is formed by asputtering method using an In—Ga—Zn metal oxide target (having an atomicratio of In:Ga:Zn=1:1:1.2), a mask is formed over the oxidesemiconductor film through a lithography process, and the oxidesemiconductor film is processed into a desired shape, whereby the oxidesemiconductor film 108 having an island shape is formed.

After the oxide semiconductor film 108 is formed, heat treatment may beperformed at a temperature higher than or equal to 150° C. and lowerthan the strain point of the substrate, preferably higher than or equalto 200° C. and lower than or equal to 450° C., or further preferablyhigher than or equal to 300° C. and lower than or equal to 450° C. Theheat treatment performed here serves as one kind of treatment forincreasing the purity of the oxide semiconductor film and can reducehydrogen, water, and the like contained in the oxide semiconductor film108. Note that the heat treatment for the purpose of reducing hydrogen,water, and the like may be performed before the oxide semiconductor film108 is processed into an island shape.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment performed on the oxide semiconductor film 108. With theuse of an RTA apparatus, the heat treatment can be performed at atemperature higher than or equal to the strain point of the substrate ifthe heating time is short. Therefore, the heat treatment time can beshortened.

Note that the heat treatment performed on the oxide semiconductor film108 may be performed under an atmosphere of nitrogen, oxygen, ultra-dryair (air in which a water content is 20 ppm or less, preferably 1 ppm orless, or further preferably 10 ppb or less), or a rare gas (argon,helium, or the like). The atmosphere of nitrogen, oxygen, ultra-dry air,or a rare gas preferably does not contain hydrogen, water, and the like.Furthermore, after heat treatment performed in a nitrogen atmosphere ora rare gas atmosphere, heat treatment may be additionally performed inan oxygen atmosphere or an ultra-dry air atmosphere. As a result,hydrogen, water, and the like can be released from the oxidesemiconductor film and oxygen can be supplied to the oxide semiconductorfilm at the same time. Consequently, the number of oxygen vacancies inthe oxide semiconductor film can be reduced.

In the case where the oxide semiconductor film 108 is formed by asputtering method, as a sputtering gas, a rare gas (typically argon),oxygen, or a mixed gas of a rare gas and oxygen is used as appropriate.In the case of using the mixed gas of a rare gas and oxygen, theproportion of oxygen to a rare gas is preferably increased. In addition,increasing the purity of a sputtering gas is necessary. For example, asan oxygen gas or an argon gas used for a sputtering gas, a gas which ishighly purified to have a dew point of −40° C. or lower, preferably −80°C. or lower, further preferably −100° C. or lower, or still furtherpreferably −120° C. or lower is used, whereby entry of moisture or thelike into the oxide semiconductor film 108 can be minimized.

In the case where the oxide semiconductor film 108 is formed by asputtering method, a chamber in a sputtering apparatus is preferablyevacuated to be a high vacuum state (to the degree of about 5×10⁻⁷ Pa to1×10⁻⁴ Pa) with an adsorption vacuum evacuation pump such as a cryopumpin order to remove water or the like, which serves as an impurity forthe oxide semiconductor film 108, as much as possible. Alternatively, aturbo molecular pump and a cold trap are preferably combined so as toprevent a backflow of a gas, especially a gas including carbon orhydrogen from an exhaust system to the inside of the chamber.

Next, the conductive film 112 is formed over the insulating film 106 band the conductive films 112 a and 112 b functioning as a sourceelectrode and a drain electrode are formed over the insulating film 106b and the oxide semiconductor film 108 (see FIG. 9C).

In this embodiment, the conductive film 112 and the conductive films 112a and 112 b are formed in the following manner: a stack formed of a50-nm-thick tungsten film and a 400-nm-thick aluminum film is formed bya sputtering method, a mask is formed over the stack through alithography process, and the stack is processed into desired shapes.Although the conductive film 112 and the conductive films 112 a and 112b each have a two-layer structure in this embodiment, one embodiment ofthe present invention is not limited thereto. For example, theconductive film 112 and the conductive films 112 a and 112 b each mayhave a three-layer structure formed of a 50-nm-thick tungsten film, a400-nm-thick aluminum film, and a 100-nm-thick titanium film.

After the conductive film 112 and the conductive films 112 a and 112 bare formed, a surface of the oxide semiconductor film 108 (on a backchannel side) may be cleaned. The cleaning may be performed, forexample, using a chemical solution such as phosphoric acid. The cleaningusing a chemical solution such as a phosphoric acid can removeimpurities (e.g., an element included in the conductive film 112 and theconductive films 112 a and 112 b) attached to the surface of the oxidesemiconductor film 108.

Note that a recessed portion might be formed in part of the oxidesemiconductor film 108 at the step of forming the conductive film 112and the conductive films 112 a and 112 b and/or the cleaning step.

Through the steps, the transistor 100 is formed.

Next, the insulating films 114 and 116 are formed over the insulatingfilm 106 b and the conductive film 112 and over the oxide semiconductorfilm 108 and the conductive films 112 a and 112 b (see FIG. 10A).

Note that after the insulating film 114 is formed, the insulating film116 is preferably formed in succession without exposure to the air.After the insulating film 114 is formed, the insulating film 116 isformed in succession by adjusting at least one of the flow rate of asource gas, pressure, a high-frequency power, and a substratetemperature without exposure to the air, whereby the concentration ofimpurities attributed to the atmospheric component at the interfacebetween the insulating film 114 and the insulating film 116 can bereduced and oxygen in the insulating films 114 and 116 can be moved tothe oxide semiconductor film 108; accordingly, the number of oxygenvacancies in the oxide semiconductor film 108 can be reduced.

For example, as the insulating film 114, a silicon oxynitride film canbe formed by a PECVD method. In this case, a deposition gas containingsilicon and an oxidizing gas are preferably used as a source gas.Typical examples of the deposition gas containing silicon includesilane, disilane, trisilane, and silane fluoride. Examples of theoxidizing gas include dinitrogen monoxide and nitrogen dioxide. Aninsulating film containing nitrogen and having a small number of defectscan be formed as the insulating film 114 by a PECVD method under theconditions where the ratio of the oxidizing gas to the deposition gas ishigher than 20 times and lower than 100 times, or preferably higher thanor equal to 40 times and lower than or equal to 80 times and thepressure in a treatment chamber is lower than 100 Pa, or preferablylower than or equal to 50 Pa.

In this embodiment, a silicon oxynitride film is formed as theinsulating film 114 by a PECVD method under the conditions where thesubstrate 102 is held at a temperature of 220° C., silane at a flow rateof 50 sccm and dinitrogen monoxide at a flow rate of 2000 sccm are usedas a source gas, the pressure in the treatment chamber is 20 Pa, and ahigh-frequency power of 100 W at 13.56 MHz (1.6×10⁻² W/cm² as the powerdensity) is supplied to parallel-plate electrodes.

As the insulating film 116, a silicon oxide film or a silicon oxynitridefilm is formed under the conditions where the substrate placed in atreatment chamber of the PECVD apparatus that is vacuum-evacuated isheld at a temperature higher than or equal to 180° C. and lower than orequal to 280° C., or preferably higher than or equal to 200° C. andlower than or equal to 240° C., the pressure is greater than or equal to100 Pa and less than or equal to 250 Pa, or preferably greater than orequal to 100 Pa and less than or equal to 200 Pa with introduction of asource gas into the treatment chamber, and a high-frequency power ofgreater than or equal to 0.17 W/cm² and less than or equal to 0.5 W/cm²,or preferably greater than or equal to 0.25 W/cm² and less than or equalto 0.35 W/cm² is supplied to an electrode provided in the treatmentchamber.

As the deposition conditions of the insulating film 116, thehigh-frequency power having the above power density is supplied to areaction chamber having the above pressure, whereby the degradationefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted; thus, the oxygencontent in the insulating film 116 becomes higher than that in thestoichiometric composition. On the other hand, in the film formed at asubstrate temperature within the above temperature range, the bondbetween silicon and oxygen is weak, and accordingly, part of oxygen inthe film is released by heat treatment in a later step. Thus, it ispossible to form an oxide insulating film which contains oxygen inexcess of that in the stoichiometric composition and from which part ofoxygen is released by heating.

Note that the insulating film 114 functions as a protective film for theoxide semiconductor film 108 in the step of forming the insulating film116. Therefore, the insulating film 116 can be formed using thehigh-frequency power having a high power density while damage to theoxide semiconductor film 108 is reduced.

Note that in the deposition conditions of the insulating film 116, whenthe flow rate of the deposition gas containing silicon with respect tothe oxidizing gas is increased, the number of defects in the insulatingfilm 116 can be reduced. Typically, it is possible to form an oxideinsulating layer in which the number of defects is small, i.e., the spindensity of a signal that appears at g=2.001 originating from a danglingbond of silicon is lower than 6×10¹⁷ spins/cm³, preferably lower than orequal to 3×10¹⁷ spins/cm³, or further preferably lower than or equal to1.5×10¹⁷ spins/cm³ by ESR measurement. As a result, the reliability ofthe transistor can be improved.

Heat treatment may be performed after the insulating films 114 and 116are formed. The heat treatment can reduce nitrogen oxide contained inthe insulating films 114 and 116. By the heat treatment, part of oxygencontained in the insulating films 114 and 116 can be moved to the oxidesemiconductor film 108, so that the number of oxygen vacancies in theoxide semiconductor film 108 can be reduced.

The temperature of the heat treatment performed on the insulating films114 and 116 is typically higher than or equal to 150° C. and lower thanor equal to 400° C., preferably higher than or equal to 300° C. andlower than or equal to 400° C., or further preferably higher than orequal to 320° C. and lower than or equal to 370° C. The heat treatmentmay be performed under an atmosphere of nitrogen, oxygen, ultra-dry air(air in which a water content is 20 ppm or less, preferably 1 ppm orless, or further preferably 10 ppb or less), or a rare gas (argon,helium, or the like). Note that an electric furnace, an RTA apparatus,or the like can be used for the heat treatment, in which it ispreferable that hydrogen, water, and the like not be contained in thenitrogen, oxygen, ultra-dry air, or rare gas.

In this embodiment, the heat treatment is performed at 350° C. in anatmosphere of nitrogen and oxygen for 1 hour.

Next, a protective film 130 that inhibits release of oxygen is formedover the insulating film 116. Then, oxygen 141 is added to theinsulating films 114 and 116 and the oxide semiconductor film 108through the protective film 130 (see FIG. 10B).

The protective film 130 that inhibits release of oxygen contains atleast one of indium, zinc, titanium, aluminum, tungsten, tantalum, andmolybdenum. For example, a conductive material such as an alloycontaining any of the metal elements, an alloy containing any of themetal elements in combination, a metal oxide containing any of the metalelements, a metal nitride containing any of the metal elements, or ametal nitride oxide containing any of the metal elements is used.

The protective film 130 that inhibits release of oxygen can be formedusing, for example, a tantalum nitride film, a titanium film, an indiumtin oxide (ITO) film, an aluminum film, or an oxide semiconductor film(e.g., an IGZO film having an atomic ratio of In:Ga:Zn=1:4:5).

The thickness of the protective film 130 that inhibits release of oxygencan be greater than or equal to 1 nm and less than or equal to 20 nm, orgreater than or equal to 2 nm and less than or equal to 10 nm. In thisembodiment, a 5-nm-thick tantalum nitride film is used as the protectivefilm 130.

As a method for adding the oxygen 141 to the insulating films 114 and116 and the oxide semiconductor film 108 through the protective film130, an ion doping method, an ion implantation method, plasma treatment,or the like is given. When the protective film 130 is provided over theinsulating film 116 and then oxygen is added, the protective film 130serves as a protective film for inhibiting oxygen from being releasedfrom the insulating film 116. Thus, a larger amount of oxygen can beadded to the insulating films 114 and 116 and the oxide semiconductorfilm 108. Note that in the case where the insulating films 114 and 116and the oxide semiconductor film 108 after deposition contain oxygen inexcess of that in the stoichiometric composition, the oxygen 141 is notnecessarily added to the insulating films 114 and 116 and the oxidesemiconductor film 108.

In the case where oxygen is introduced by plasma treatment, by makingoxygen excited by a microwave to generate high density oxygen plasma,the amount of oxygen introduced into the insulating film 116 can beincreased.

After that, the protective film 130 is removed, and the insulating film118 is formed over the insulating film 116 (see FIG. 11A).

Note that by the addition of the oxygen 141, the protective film 130becomes the insulating film formed of oxide or nitride of metal (indium,zinc, titanium, aluminum, tungsten, tantalum, or molybdenum). Althoughthe method for forming the insulating film 118 after removal of theprotective film 130 is described as an example in this embodiment,without limitation thereto, the insulating film 118 may be formed overthe protective film 130 without removal of the protective film 130.

Note that heat treatment may be performed before or after the formationof the insulating film 118, so that excess oxygen contained in theinsulating films 114 and 116 can be diffused into the oxidesemiconductor film 108 to fill an oxygen vacancy in the oxidesemiconductor film 108. Alternatively, the insulating film 118 may bedeposited by heating, so that excess oxygen contained in the insulatingfilms 114 and 116 can be diffused into the oxide semiconductor film 108to fill an oxygen vacancy in the oxide semiconductor film 108.

In the case where the insulating film 118 is formed by a PECVD method,the substrate temperature is preferably set to higher than or equal to300° C. and lower than or equal to 400° C., or further preferably higherthan or equal to 320° C. and lower than or equal to 370° C., so that adense film can be formed.

For example, in the case where a silicon nitride film is formed by aPECVD method as the insulating film 118, a deposition gas containingsilicon, nitrogen, and ammonia are preferably used as a source gas. Asmall amount of ammonia compared to the amount of nitrogen is used,whereby ammonia is dissociated in the plasma and activated species aregenerated. The activated species cleave a bond between silicon andhydrogen which are included in a deposition gas containing silicon and atriple bond between nitrogen molecules. As a result, a dense siliconnitride film having few defects, in which bonds between silicon andnitrogen are promoted and bonds between silicon and hydrogen is few, canbe formed. On the other hand, when the amount of ammonia with respect tonitrogen is large, decomposition of a deposition gas containing siliconand decomposition of nitrogen are not promoted, so that a sparse siliconnitride film in which bonds between silicon and hydrogen remain anddefects are increased is formed. Therefore, in the source gas, a flowrate ratio of the nitrogen to the ammonia is set to be greater than orequal to 5 and less than or equal to 50, or preferably greater than orequal to 10 and less than or equal to 50.

In this embodiment, with the use of a PECVD apparatus, a 50-nm-thicksilicon nitride film is formed as the insulating film 118 using silane,nitrogen, and ammonia as a source gas. The flow rate of silane is 50sccm, the flow rate of nitrogen is 5000 sccm, and the flow rate ofammonia is 100 sccm. The pressure in the treatment chamber is 100 Pa,the substrate temperature is 350° C., and high-frequency power of 1000 Wis supplied to parallel-plate electrodes with a 27.12 MHz high-frequencypower source. Note that the PECVD apparatus is a parallel-plate PECVDapparatus in which the electrode area is 6000 cm², and the power perunit area (power density) into which the supplied power is converted is1.7×10⁻¹ W/cm².

Heat treatment may be performed after the formation of the insulatingfilm 118. The heat treatment is performed typically at a temperaturehigher than or equal to 150° C. and lower than or equal to 400° C.,preferably higher than or equal to 300° C. and lower than or equal to400° C., or further preferably higher than or equal to 320° C. and lowerthan or equal to 370° C. When the heat treatment is performed, theamounts of hydrogen and water in the insulating films 114 and 116 arereduced and accordingly the generation of defects in the oxidesemiconductor film 108 described above is inhibited.

Next, the opening 142 reaching the conductive film 104 is formed bypartial removal of the insulating films 106 a and 106 b and theinsulating films 114, 116, and 118. In addition, the opening 142 areaching the conductive film 112 b is formed by partial removal of theinsulating films 114, 116, and 118 (see FIG. 11B).

As a method for forming the openings 142 and 142 a, a mask is formedover the insulating film 118 through a lithography process, and desiredregions of the insulating films 106 a and 106 b and the insulating films114, 116, and 118 are processed. The openings 142 and 142 a may beformed using, for example, a gray-tone mask or a half-tone mask.Although the method for forming the openings 142 and 142 a in the samestep is described as an example in this embodiment, without limitationthereto, the openings 142 and 142 a may be formed in different steps,for example.

Next, a conductive film is formed over the insulating film 118, theconductive film 104, and the conductive film 112 b so as to cover theopenings 142 and 142 a, and the conductive film is processed into adesired shape, so that the conductive films 120 and 120 a are formed(see FIG. 12A).

For the conductive films 120 and 120 a, a light-transmitting conductivematerial such as indium oxide including tungsten oxide, indium zincoxide containing tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium tin oxide, indiumzinc oxide, or indium tin oxide to which silicon oxide is added can beused. The conductive films 120 and 120 a can be formed by a sputteringmethod, for example. In this embodiment, a 110-nm-thick indium tin oxidefilm to which silicon oxide is added is formed with a sputteringapparatus.

Next, an insulating film is formed over the conductive films 120 and 120a and is processed into a desired shape, so that the insulating film 122is formed (see FIG. 12B).

In this embodiment, with the use of a PECVD apparatus, a 100-nm-thicksilicon nitride film is formed as the insulating film 122 using silaneand nitrogen as a source gas. The flow rate of silane is 200 sccm, andthe flow rate of nitrogen is 5000 sccm. The pressure in the treatmentchamber is 100 Pa, the substrate temperature is 350° C., andhigh-frequency power of 2000 W is supplied to parallel-plate electrodeswith a 27.12 MHz high-frequency power source.

As described above, an ammonia gas is not used as a source gas forformation of the insulating film 122, whereby the amount of ammonia gasreleased from the insulating film 122 can be suppressed.

Through the above process, the semiconductor device illustrated in FIGS.1A and 7B and FIGS. 3A to 3C can be manufactured.

<Method 2 for Manufacturing Semiconductor Device>

Next, a method for manufacturing the transistor 150 in FIGS. 4A to 4Cthat is a semiconductor device of one embodiment of the presentinvention is described below in detail with reference to FIGS. 13A to13D, FIGS. 14A to 14C, and FIGS. 15A and 15B. Note that FIGS. 13A to13D, FIGS. 14A to 14C, and FIGS. 15A and 15B are cross sectional viewsillustrating the manufacturing method of the transistor 150.

First, a step similar to the step in FIG. 9B is performed, and then theinsulating films 114 and 116 and the protective film 130 that inhibitsrelease of oxygen are formed over the oxide semiconductor film 108 (seeFIG. 13A).

Next, the oxygen 141 is added to the insulating films 114 and 116 andthe oxide semiconductor film 108 through the protective film 130 (seeFIG. 13B).

Next, the protective film 130 is removed, so that the insulating film116 is exposed (see FIG. 13C).

Next, a mask is formed over the insulating film 116 through alithography process, and the openings 141 a and 141 b are formed indesired regions in the insulating films 114 and 116. Note that theopenings 141 a and 141 b reach the oxide semiconductor film 108 (seeFIG. 13D).

Next, a conductive film is formed over the oxide semiconductor film 108and the insulating film 116 to cover the openings 141 a and 141 b, amask is formed over the conductive film through a lithography process,and the conductive film is processed into desired shapes, whereby theconductive films 112 a and 112 b are formed (see FIG. 14A).

Next, the insulating film 118 is formed over the insulating film 116 andthe conductive films 112 a and 112 b (see FIG. 14B).

Next, a mask is formed over the insulating film 118 through alithography process, and the opening 142 b is formed in a desired regionin the insulating film 118. Note that the opening 142 b reaches theconductive film 112 b (see FIG. 14C).

Next, a conductive film is formed over the insulating film 118 and theconductive film 112 b so as to cover the opening 142 b, and theconductive film is processed into a desired shape, so that theconductive film 120 a is formed (see FIG. 15A).

Next, an insulating film is formed over the insulating film 118 and theconductive film 120 a and is processed into a desired shape, so that theinsulating film 122 is formed. Note that the insulating film 122 coversthe end portion of the conductive film 120 a (see FIG. 15B).

Through the above process, the transistor 150 illustrated in FIGS. 4A to4C can be manufactured.

Note that the transistor 160 in FIGS. 5A to 5C can be manufactured insuch a manner that the insulating films 114 and 116 are processed intoan island shape over a channel region of the oxide semiconductor film108 at the formation of the openings 141 a and 141 b in FIG. 13D andthen through the same steps as the transistor 150 in FIGS. 4A to 4C.

<Method 3 for Manufacturing Semiconductor Device>

Next, a method for manufacturing the transistor 170 that is asemiconductor device of one embodiment of the present invention isdescribed below in detail with reference to FIGS. 16A to 16D and FIGS.17A to 17D.

FIGS. 16A and 16C and FIGS. 17A and 17C are each a cross-sectional viewof the manufacturing method in the channel length direction of thetransistor 170 and FIGS. 16B and 16D and FIGS. 17B and 17D are each across-sectional view of the manufacturing method in the channel widthdirection of the transistor 170.

First, through a step similar to the step in FIG. 11A, the insulatingfilm 118 is formed over the insulating film 116 (see FIGS. 16A and 16B).

Next, a mask is formed over the insulating film 118 through alithography process, and the opening 142 a is formed in a desired regionin the insulating films 114, 116, and 118. In addition, a mask is formedover the insulating film 118 through a lithography process, and theopenings 142 c and 142 d are formed in desired regions in the insulatingfilms 106 a, 106 b, 114, 116, and 118. Note that the opening 142 areaches the conductive film 112 b. The openings 142 c and 142 d eachreach the conductive film 104 a (see FIGS. 16C and 16D).

Note that the openings 142 c and 142 d and the opening 142 a may beformed at a time or may be formed by different steps. In the case wherethe openings 142 c and 142 d and the opening 142 a are formed at a time,for example, a gray-tone mask or a half-tone mask may be used.

Next, a conductive film is formed over the insulating film 118 so as tocover the openings 142 a, 142 c, and 142 d, and the conductive film isprocessed into a desired shape, so that the conductive film 120 a isformed (see FIGS. 17A and 17B).

Next, an insulating film is formed over the insulating film 118 and theconductive films 120 a and 120 b and is processed into a desired shape,so that the insulating film 122 is formed (see FIGS. 17C and 17D).

Through the above process, the transistor 170 illustrated in FIGS. 6A to6C can be manufactured.

Embodiment 2

In this embodiment, the structure of an oxide semiconductor filmincluded in a semiconductor device of one embodiment of the presentinvention will be described below in detail.

<Oxide Semiconductor Structure>

First, a structure of an oxide semiconductor is described.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofa non-single-crystal oxide semiconductor include a c-axis alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

From another perspective, an oxide semiconductor is classified into anamorphous oxide semiconductor and a crystalline oxide semiconductor. Inaddition, examples of a crystalline oxide semiconductor include a singlecrystal oxide semiconductor, a CAAC-OS, a polycrystalline oxidesemiconductor, and an nc-OS.

It is known that an amorphous structure is generally defined as beingmetastable and unfixed, and being isotropic and having no non-uniformstructure. In other words, an amorphous structure has a flexible bondangle and a short-range order but does not have a long-range order.

This means that an inherently stable oxide semiconductor cannot beregarded as a completely amorphous oxide semiconductor. Moreover, anoxide semiconductor that is not isotropic (e.g., an oxide semiconductorfilm that has a periodic structure in a microscopic region) cannot beregarded as a completely amorphous oxide semiconductor. Note that ana-like OS has a periodic structure in a microscopic region, but at thesame time has a void and has an unstable structure. For this reason, ana-like OS has physical properties similar to those of an amorphous oxidesemiconductor.

<CAAC-OS>

First, a CAAC-OS is described.

A CAAC-OS is one of oxide semiconductors having a plurality of c-axisaligned crystal parts (also referred to as pellets).

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OS,which is obtained using a transmission electron microscope (TEM), aplurality of pellets can be observed. However, in the high-resolutionTEM image, a boundary between pellets, that is, a grain boundary is notclearly observed. Thus, in the CAAC-OS, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

The CAAC-OS observed with a TEM is described below. FIG. 18A shows ahigh-resolution TEM image of a cross section of the CAAC-OS which isobserved from a direction substantially parallel to the sample surface.The high-resolution TEM image is obtained with a spherical aberrationcorrector function. The high-resolution TEM image obtained with aspherical aberration corrector function is particularly referred to as aCs-corrected high-resolution TEM image. The Cs-corrected high-resolutionTEM image can be obtained with, for example, an atomic resolutionanalytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.

FIG. 18B is an enlarged Cs-corrected high-resolution TEM image of aregion (1) in FIG. 18A. FIG. 18B shows that metal atoms are arranged ina layered manner in a pellet. Each metal atom layer has a configurationreflecting unevenness of a surface over which a CAAC-OS film is formed(hereinafter, the surface is referred to as a formation surface) or atop surface of the CAAC-OS, and is arranged parallel to the formationsurface or the top surface of the CAAC-OS.

As shown in FIG. 18B, the CAAC-OS has a characteristic atomicarrangement. The characteristic atomic arrangement is denoted by anauxiliary line in FIG. 18C. FIGS. 18B and 18C prove that the size of apellet is approximately 1 nm to 3 nm, and the size of a space caused bytilt of the pellets is approximately 0.8 nm. Therefore, the pellet canalso be referred to as a nanocrystal (nc). Furthermore, a CAAC-OS can bereferred to as an oxide semiconductor including c-axis alignednanocrystals (CANC).

Here, according to the Cs-corrected high-resolution TEM images, theschematic arrangement of pellets 5100 of a CAAC-OS over a substrate 5120is illustrated by such a structure in which bricks or blocks are stacked(see FIG. 18D). The part in which the pellets are tilted as observed inFIG. 18C corresponds to a region 5161 shown in FIG. 18D.

FIG. 19A shows a Cs-corrected high-resolution TEM image of a plane ofthe CAAC-OS observed from a direction substantially perpendicular to thesample surface. FIGS. 19B, 19C, and 19D are enlarged Cs-correctedhigh-resolution TEM images of regions (1), (2), and (3) in FIG. 19A,respectively. FIGS. 19B, 19C, and 19D indicate that metal atoms arearranged in a triangular, quadrangular, or hexagonal configuration in apellet. However, there is no regularity of arrangement of metal atomsbetween different pellets.

Next, a CAAC-OS analyzed by X-ray diffraction (XRD) is described. Forexample, when the structure of a CAAC-OS including an InGaZnO₄ crystalis analyzed by an out-of-plane method, a peak appears at a diffractionangle (2θ) of around 31° as shown in FIG. 20A. This peak is derived fromthe (009) plane of the InGaZnO₄ crystal, which indicates that crystalsin the CAAC-OS have c-axis alignment, and that the c-axes are aligned ina direction substantially perpendicular to the formation surface or thetop surface of the CAAC-OS.

Note that in structural analysis of the CAAC-OS by an out-of-planemethod, another peak may appear when 2θ is around 36°, in addition tothe peak at 2θ of around 31°. The peak of 2θ at around 36° indicatesthat a crystal having no c-axis alignment is included in part of theCAAC-OS. It is preferable that in the CAAC-OS analyzed by anout-of-plane method, a peak appear when 2θ is around 31° and that a peaknot appear when 2θ is around 36°.

On the other hand, in structural analysis of the CAAC-OS by an in-planemethod in which an X-ray is incident on a sample in a directionsubstantially perpendicular to the c-axis, a peak appears when 2θ isaround 56°. This peak is derived from the (110) plane of the InGaZnO₄crystal. In the case of the CAAC-OS, when analysis (ϕ scan) is performedwith 2θ fixed at around 56° and with the sample rotated using a normalvector of the sample surface as an axis (ϕ axis), as shown in FIG. 20B,a peak is not clearly observed. In contrast, in the case of a singlecrystal oxide semiconductor of InGaZnO₄, when ϕ scan is performed with2θ fixed at around 56°, as shown in FIG. 20C, six peaks which arederived from crystal planes equivalent to the (110) plane are observed.Accordingly, the structural analysis using XRD shows that the directionsof a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. Forexample, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in a directionparallel to the sample surface, a diffraction pattern (also referred toas a selected-area transmission electron diffraction pattern) shown inFIG. 21A can be obtained. In this diffraction pattern, spots derivedfrom the (009) plane of an InGaZnO₄ crystal are included. Thus, theelectron diffraction also indicates that pellets included in the CAAC-OShave c-axis alignment and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS. Meanwhile, FIG. 21B shows a diffraction pattern obtainedin such a manner that an electron beam with a probe diameter of 300 nmis incident on the same sample in a direction perpendicular to thesample surface. As shown in FIG. 21B, a ring-like diffraction pattern isobserved. Thus, the electron diffraction also indicates that the a-axesand b-axes of the pellets included in the CAAC-OS do not have regularalignment. The first ring in FIG. 21B is considered to be derived fromthe (010) plane, the (100) plane, and the like of the InGaZnO₄ crystal.Furthermore, it is supposed that the second ring in FIG. 21B is derivedfrom the (110) plane and the like.

As described above, the CAAC-OS is an oxide semiconductor with highcrystallinity. Entry of impurities, formation of defects, or the likemight decrease the crystallinity of an oxide semiconductor. This meansthat the CAAC-OS has small amounts of impurities and defects (e.g.,oxygen vacancies).

Note that the impurity means an element other than the main componentsof the oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. For example, an element (specifically, siliconor the like) having higher strength of bonding to oxygen than a metalelement included in an oxide semiconductor extracts oxygen from theoxide semiconductor, which results in disorder of the atomic arrangementand reduced crystallinity of the oxide semiconductor. A heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disturbs the atomic arrangementof the oxide semiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities ordefects might be changed by light, heat, or the like. Impuritiescontained in the oxide semiconductor might serve as carrier traps orcarrier generation sources, for example. Furthermore, an oxygen vacancyin the oxide semiconductor serves as a carrier trap or serves as acarrier generation source when hydrogen is captured therein.

The CAAC-OS having small numbers of impurities and oxygen vacancies isan oxide semiconductor film with low carrier density (specifically,lower than 8×10¹¹/cm³, preferably lower than 1×10¹¹/cm³, or furtherpreferably lower than 1×10¹⁰/cm³, and is higher than or equal to1×10⁻⁹/cm³). Such an oxide semiconductor is referred to as a highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor. A CAAC-OS has a low impurity concentration and a lowdensity of defect states. Thus, the CAAC-OS can be referred to as anoxide semiconductor having stable characteristics.

<nc-OS>

Next, an nc-OS is described.

An nc-OS has a region in which a crystal part is observed and a regionin which a crystal part is not clearly observed in a high-resolution TEMimage. In most cases, the size of a crystal part included in the nc-OSis greater than or equal to 1 nm and less than or equal to 10 nm, orgreater than or equal to 1 nm and less than or equal to 3 nm. Note thatan oxide semiconductor including a crystal part whose size is greaterthan 10 nm and less than or equal to 100 nm is sometimes referred to asa microcrystalline oxide semiconductor. In a high-resolution TEM imageof the nc-OS, for example, a grain boundary is not clearly observed insome cases. Note that there is a possibility that the origin of thenanocrystal is the same as that of a pellet in a CAAC-OS. Therefore, acrystal part of the nc-OS may be referred to as a pellet in thefollowing description.

In the nc-OS, a microscopic region (e.g., a region with a size greaterthan or equal to 1 nm and less than or equal to 10 nm, in particular, aregion with a size greater than or equal to 1 nm and less than or equalto 3 nm) has a periodic atomic arrangement. There is no regularity ofcrystal orientation between different pellets in the nc-OS. Thus, theorientation of the whole film is not observed. Accordingly, the nc-OScannot be distinguished from an a-like OS and an amorphous oxidesemiconductor, depending on an analysis method. For example, when thenc-OS is analyzed by an out-of-plane method using an X-ray beam having adiameter larger than the size of a pellet, a peak which shows a crystalplane does not appear. Furthermore, a diffraction pattern like a halopattern is observed when the nc-OS is subjected to electron diffractionusing an electron beam with a probe diameter (e.g., 50 nm or larger)that is larger than the size of a pellet. Meanwhile, spots appear in ananobeam electron diffraction pattern of the nc-OS when an electron beamhaving a probe diameter close to or smaller than the size of a pellet isapplied. Moreover, in a nanobeam electron diffraction pattern of thenc-OS, regions with high luminance in a circular (ring) pattern areshown in some cases. Also in a nanobeam electron diffraction pattern ofthe nc-OS layer, a plurality of spots is shown in a ring-like region insome cases.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS can also be referred to asan oxide semiconductor including random aligned nanocrystals (RANC) oran oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as comparedwith an amorphous oxide semiconductor. Therefore, the nc-OS is likely tohave a lower density of defect states than an a-like OS and an amorphousoxide semiconductor. Note that there is no regularity of crystalorientation between different pellets in the nc-OS. Therefore, the nc-OShas a higher density of defect states than the CAAC-OS.

<a-Like OS>

An a-like OS has a structure intermediate between those of the nc-OS andthe amorphous oxide semiconductor.

In a high-resolution TEM image of the a-like OS, a void may be observed.Furthermore, in the high-resolution TEM image, there are a region wherea crystal part is clearly observed and a region where a crystal part isnot observed.

The a-like OS has an unstable structure because it includes a void. Toverify that an a-like OS has an unstable structure as compared with aCAAC-OS and an nc-OS, a change in structure caused by electronirradiation is described below.

An a-like OS (sample A), an nc-OS (sample B), and a CAAC-OS (sample C)are prepared as samples subjected to electron irradiation. Each of thesamples is an In—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

Note that which part is regarded as a crystal part is determined asfollows. It is known that a unit cell of the InGaZnO₄ crystal has astructure in which nine layers including three In—O layers and sixGa—Zn—O layers are stacked in the c-axis direction. The distance betweenthe adjacent layers is equivalent to the lattice spacing on the (009)plane (also referred to as d value). The value is calculated to be 0.29nm from crystal structural analysis. Accordingly, a portion where thelattice spacing between lattice fringes is greater than or equal to 0.28nm and less than or equal to 0.30 nm is regarded as a crystal part ofInGaZnO₄. Each of lattice fringes corresponds to the a-b plane of theInGaZnO₄ crystal.

FIG. 33 shows change in the average size of crystal parts (at 22 pointsto 45 points) in each sample. Note that the crystal part sizecorresponds to the length of a lattice fringe. FIG. 33 indicates thatthe crystal part size in the a-like OS increases with an increase in thecumulative electron dose. Specifically, as shown by (1) in FIG. 33, acrystal part of approximately 1.2 nm at the start of TEM observation(the crystal part is also referred to as an initial nucleus) grows to asize of approximately 2.6 nm at a cumulative electron dose of 4.2×10⁸e⁻/nm². In contrast, the crystal part size in the nc-OS and the CAAC-OSshows little change from the start of electron irradiation to acumulative electron dose of 4.2×10⁸ e⁻/nm². Specifically, as shown by(2) and (3) in FIG. 33, the average crystal sizes in an nc-OS and aCAAC-OS are approximately 1.4 nm and approximately 2.1 nm, respectively,regardless of the cumulative electron dose.

In this manner, growth of the crystal part in the a-like OS is inducedby electron irradiation. In contrast, in the nc-OS and the CAAC-OS,growth of the crystal part is hardly induced by electron irradiation.Therefore, the a-like OS has an unstable structure as compared with thenc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS becauseit includes a void. Specifically, the density of the a-like OS is higherthan or equal to 78.6% and lower than 92.3% of the density of the singlecrystal oxide semiconductor having the same composition. The density ofeach of the nc-OS and the CAAC-OS is higher than or equal to 92.3% andlower than 100% of the density of the single crystal oxide semiconductorhaving the same composition. Note that it is difficult to deposit anoxide semiconductor having a density of lower than 78% of the density ofthe single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomicratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, thedensity of the a-like OS is higher than or equal to 5.0 g/cm³ and lowerthan 5.9 g/cm³. For example, in the case of the oxide semiconductorhaving an atomic ratio of In:Ga:Zn=1:1:1, the density of each of thenc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lowerthan 6.3 g/cm³.

Note that there is a possibility that an oxide semiconductor having acertain composition cannot exist in a single crystal structure. In thatcase, single crystal oxide semiconductors with different compositionsare combined at an adequate ratio, which makes it possible to calculatedensity equivalent to that of a single crystal oxide semiconductor withthe desired composition. The density of a single crystal oxidesemiconductor having the desired composition can be calculated using aweighted average according to the combination ratio of the singlecrystal oxide semiconductors with different compositions. Note that itis preferable to use as few kinds of single crystal oxide semiconductorsas possible to calculate the density.

As described above, oxide semiconductors have various structures andvarious properties. Note that an oxide semiconductor may be a stackedlayer including two or more of an amorphous oxide semiconductor, ana-like OS, an nc-OS, and a CAAC-OS, for example.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Embodiment 3

In this embodiment, an example of a display device that includes any ofthe transistors described in the embodiment above will be describedbelow with reference to FIG. 22, FIG. 23, and FIG. 24.

FIG. 22 is a top view of an example of a display device. A displaydevice 700 illustrated in FIG. 22 includes a pixel portion 702 providedover a first substrate 701; a source driver circuit portion 704 and agate driver circuit portion 706 provided over the first substrate 701; asealant 712 provided to surround the pixel portion 702, the sourcedriver circuit portion 704, and the gate driver circuit portion 706; anda second substrate 705 provided to face the first substrate 701. Thefirst substrate 701 and the second substrate 705 are sealed with thesealant 712. That is, the pixel portion 702, the source driver circuitportion 704, and the gate driver circuit portion 706 are sealed with thefirst substrate 701, the sealant 712, and the second substrate 705.Although not illustrated in FIG. 22, a display element is providedbetween the first substrate 701 and the second substrate 705.

In the display device 700, a flexible printed circuit (FPC) terminalportion 708 electrically connected to the pixel portion 702, the sourcedriver circuit portion 704, and the gate driver circuit portion 706 isprovided in a region different from the region which is surrounded bythe sealant 712 and positioned over the first substrate 701.Furthermore, an FPC 716 is connected to the FPC terminal portion 708,and a variety of signals and the like are supplied to the pixel portion702, the source driver circuit portion 704, and the gate driver circuitportion 706 through the FPC 716. Furthermore, a signal line 710 isconnected to the pixel portion 702, the source driver circuit portion704, the gate driver circuit portion 706, and the FPC terminal portion708. Various signals and the like are applied to the pixel portion 702,the source driver circuit portion 704, the gate driver circuit portion706, and the FPC terminal portion 708 via the signal line 710 from theFPC 716.

A plurality of gate driver circuit portions 706 may be provided in thedisplay device 700. An example of the display device 700 in which thesource driver circuit portion 704 and the gate driver circuit portion706 are formed over the first substrate 701 where the pixel portion 702is also formed is described; however, the structure is not limitedthereto. For example, only the gate driver circuit portion 706 may beformed over the first substrate 701 or only the source driver circuitportion 704 may be formed over the first substrate 701. In this case, asubstrate where a source driver circuit, a gate driver circuit, or thelike is formed (e.g., a driver-circuit substrate formed using asingle-crystal semiconductor film or a polycrystalline semiconductorfilm) may be mounted on the first substrate 701. Note that there is noparticular limitation on the method of connecting a separately prepareddriver circuit substrate, and a chip on glass (COG) method, a wirebonding method, or the like can be used.

The pixel portion 702, the source driver circuit portion 704, and thegate driver circuit portion 706 included in the display device 700include a wiring portion or a plurality of transistors. As the wiringportion or the plurality of transistors, any of the semiconductordevices of embodiments of the present invention can be used.

The display device 700 can include any of a variety of elements. Theelement includes, for example, at least one of a liquid crystal element,an electroluminescence (EL) element (e.g., an EL element includingorganic and inorganic materials, an organic EL element, or an inorganicEL element), an LED (e.g., a white LED, a red LED, a green LED, or ablue LED), a transistor (a transistor that emits light depending oncurrent), an electron emitter, electronic ink, an electrophoreticelement, a grating light valve (GLV), a plasma display panel (PDP), adisplay element using micro electro mechanical system (MEMS), a digitalmicromirror device (DMD), a digital micro shutter (DMS), MIRASOL(registered trademark), an interferometric modulator display (IMOD)element, a MEMS shutter display element, an optical-interference-typeMEMS display element, an electrowetting element, a piezoelectric ceramicdisplay, and a display element including a carbon nanotube. Other thanthe above, display media whose contrast, luminance, reflectivity,transmittance, or the like is changed by an electrical or magneticeffect may be included. Examples of display devices having EL elementsinclude an EL display. Examples of display devices including electronemitters include a field emission display (FED) and an SED-type flatpanel display (SED: surface-conduction electron-emitter display).Examples of display devices including liquid crystal elements include aliquid crystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display). An example of a display device including electronicink or electrophoretic elements is electronic paper. In the case of atransflective liquid crystal display or a reflective liquid crystaldisplay, some of or all of pixel electrodes function as reflectiveelectrodes. For example, some or all of pixel electrodes are formed toinclude aluminum, silver, or the like. In such a case, a memory circuitsuch as an SRAM can be provided under the reflective electrodes, leadingto lower power consumption.

As a display method in the display device 700, a progressive method, aninterlace method, or the like can be employed. Furthermore, colorelements controlled in a pixel at the time of color display are notlimited to three colors: R, G, and B (R, G, and B correspond to red,green, and blue, respectively). For example, four pixels of the R pixel,the G pixel, the B pixel, and a W (white) pixel may be included.Alternatively, a color element may be composed of two colors among R, G,and B as in PenTile layout. The two colors may differ among colorelements. Alternatively, one or more colors of yellow, cyan, magenta,and the like may be added to RGB. Furthermore, the size of a displayregion may be different depending on respective dots of the colorcomponents. Embodiments of the disclosed invention are not limited to adisplay device for color display; the disclosed invention can also beapplied to a display device for monochrome display.

A coloring layer (also referred to as a color filter) may be used inorder to obtain a full-color display device in which white light (W) fora backlight (e.g., an organic EL element, an inorganic EL element, anLED, or a fluorescent lamp) is used. As the coloring layer, red (R),green (G), blue (B), yellow (Y), or the like may be combined asappropriate, for example. With the use of the coloring layer, highercolor reproducibility can be obtained than in the case without thecoloring layer. In this case, by providing a region with the coloringlayer and a region without the coloring layer, white light in the regionwithout the coloring layer may be directly utilized for display. Bypartly providing the region without the coloring layer, a decrease inluminance due to the coloring layer can be suppressed, and 20% to 30% ofpower consumption can be reduced in some cases when an image isdisplayed brightly. Note that in the case where full-color display isperformed using a self-luminous element such as an organic EL element oran inorganic EL element, elements may emit light of their respectivecolors R, G, B, Y, and W. By using a self-luminous element, powerconsumption can be further reduced as compared to the case of using thecoloring layer in some cases.

In this embodiment, a structure including a liquid crystal element andan EL element as display elements is described with reference to FIG. 23and FIG. 24. Note that FIG. 23 is a cross-sectional view taken along thedashed-dotted line Q-R in FIG. 22 and shows a structure including aliquid crystal element as a display element, whereas FIG. 24 is across-sectional view taken along the dashed-dotted line Q-R in FIG. 22and shows a structure including an EL element as a display element.

Common portions between FIG. 23 and FIG. 24 are described first, andthen different portions are described.

<Common Portions in Display Devices>

The display device 700 illustrated in FIG. 23 and FIG. 24 include a leadwiring portion 711, the pixel portion 702, the source driver circuitportion 704, and the FPC terminal portion 708. Note that the lead wiringportion 711 includes a signal line 710. The pixel portion 702 includes atransistor 750 and a capacitor 790 (a capacitor 790 a or 790 b). Thesource driver circuit portion 704 includes a transistor 752.

The semiconductor device in FIGS. 1A and 1B or FIGS. 2A and 2B can beused for the lead wiring portion 711. Note that in FIG. 23 and FIG. 24,only the signal line 710 is illustrated to avoid complexity.

The signal line 710 is formed in the same process as conductive filmsfunctioning as a source electrode and a drain electrode of thetransistor 750 or 752. Note that the signal line 710 may be formed usinga conductive film which is formed in a different process as a sourceelectrode and a drain electrode of the transistor 750 or 752, forexample, a conductive film functioning as a gate electrode may be used.In the case where the signal line 710 is formed using a materialincluding a copper element, signal delay or the like due to wiringresistance is reduced, which enables display on a large screen.

Any of the transistors described above can be used as the transistors750 and 752.

The transistors used in this embodiment each include an oxidesemiconductor film which is highly purified and which suppressesformation of an oxygen vacancy. In the transistor, the current in an offstate (off-state current) can be made small. Accordingly, an electricalsignal such as an image signal can be held for a longer period, and awriting interval can be set longer in an on state. Accordingly,frequency of refresh operation can be reduced, which leads to an effectof suppressing power consumption.

In addition, the transistor used in this embodiment can have relativelyhigh field-effect mobility and thus is capable of high speed operation.For example, with such a transistor which can operate at high speed usedfor a liquid crystal display device, a switching transistor in a pixelportion and a driver transistor in a driver circuit portion can beformed over one substrate. That is, a semiconductor device formed usinga silicon wafer or the like is not additionally needed as a drivercircuit, by which the number of components of the semiconductor devicecan be reduced. In addition, the transistor which can operate at highspeed can be used also in the pixel portion, whereby a high-qualityimage can be provided.

The FPC terminal portion 708 includes a connection electrode 760, ananisotropic conductive film 780, and the FPC 716. Note that theconnection electrode 760 is formed in the same process as conductivefilms functioning as a source electrode and a drain electrode of thetransistor 750 or 752. The connection electrode 760 is electricallyconnected to a terminal included in the FPC 716 through the anisotropicconductive film 780.

For example, a glass substrate can be used as the first substrate 701and the second substrate 705. A flexible substrate may be used as thefirst substrate 701 and the second substrate 705. Examples of theflexible substrate include a plastic substrate.

Furthermore, a light-blocking film 738 functioning as a black matrix, acoloring film 736 functioning as a color filter, and an insulating film734 in contact with the light-blocking film 738 and the coloring film736 are provided on the second substrate 705 side.

A structure body 778 is provided between the first substrate 701 and thesecond substrate 705. The structure body 778 is a columnar spacerobtained by selective etching of an insulating film and provided tocontrol the distance (cell gap) between the first substrate 701 and thesecond substrate 705. Note that a spherical spacer may be used as thestructure body 778. Although the structure in which the structure body778 is provided on the second substrate 705 side is illustrated in FIG.23 as an example, one embodiment of the present invention is not limitedthereto. For example, a structure in which the structure body 778 isprovided on the first substrate 701 side as illustrated in FIG. 24, or astructure in which both of the first substrate 701 and the secondsubstrate 705 are provided with the structure body 778 may be employed.

In FIG. 23 and FIG. 24, insulating films 764, 766, 768, and 769 areformed over the transistor 750, the transistor 752, and the capacitor790.

The insulating films 764, 766, 768, and 769 can be formed usingmaterials and methods similar to those of the insulating films 114, 116,118, and 122 described in the above embodiment, respectively.

<Structure Example of Display Device Using Liquid Crystal Element asDisplay Element>

The display device 700 illustrated in FIG. 23 includes the capacitor 790a. The capacitor 790 a includes a dielectric between a pair ofelectrodes. Specifically, an oxide semiconductor film with highconductivity which is formed using steps of forming the same oxidesemiconductor film as the oxide semiconductor film functioning as asemiconductor layer of the transistor 750 is used as one electrode ofthe capacitor 790 a, and a conductive film 772 electrically connected tothe transistor 750 is used as the other electrode of the capacitor 790a.

Here, an oxide semiconductor film with high conductivity which functionsas one electrode of the capacitor 790 a is described below.

<Oxide Semiconductor Film With High Conductivity>

When hydrogen is added to an oxide semiconductor including oxygenvacancies, hydrogen enters oxygen vacant sites and forms a donor levelin the vicinity of the conduction band. As a result, the conductivity ofthe oxide semiconductor is increased, so that the oxide semiconductorbecomes a conductor. An oxide semiconductor having become a conductorcan be referred to as an oxide conductor. Oxide semiconductors generallyhave a visible light-transmitting property because of their large energygap. An oxide conductor is an oxide semiconductor having a donor levelin the vicinity of the conduction band. Therefore, the influence ofabsorption due to the donor level is small, and an oxide conductor has avisible light-transmitting property comparable to that of an oxidesemiconductor.

Here, the temperature dependence of resistivity of a film formed with anoxide semiconductor (hereinafter referred to as an oxide semiconductorfilm (OS)) and that of a film formed with an oxide conductor(hereinafter referred to as an oxide conductor film (OC)) will bedescribed with reference to FIG. 28. In FIG. 28, the horizontal axisrepresents measurement temperature, and the vertical axis representsresistivity. Measurement results of the oxide semiconductor film (OS)are plotted as circles, and measurement results of the oxide conductorfilm (OC) are plotted as squares.

Note that a sample including the oxide semiconductor film (OS) wasprepared by forming a 35-nm-thick In—Ga—Zn oxide film over a glasssubstrate by a sputtering method using a sputtering target with anatomic ratio of In:Ga:Zn=1:1:1.2, forming a 20-nm-thick In—Ga—Zn oxidefilm over the 35-nm-thick In—Ga—Zn oxide film by a sputtering methodusing a sputtering target with an atomic ratio of In:Ga:Zn=1:4:5,performing heat treatment in a 450° C. nitrogen atmosphere and thenperforming heat treatment in a 450° C. atmosphere of a mixed gas ofnitrogen and oxygen, and forming a silicon oxynitride film over the20-nm-thick In—Ga—Zn oxide film by a plasma CVD method.

A sample including the oxide conductor film (OC) was prepared by forminga 100-nm-thick In—Ga—Zn oxide film over a glass substrate by asputtering method using a sputtering target with an atomic ratio ofIn:Ga:Zn=1:1:1, performing heat treatment in a 450° C. nitrogenatmosphere and then performing heat treatment in a 450° C. atmosphere ofa mixed gas of nitrogen and oxygen, and forming a silicon nitride filmover the 100-nm-thick In—Ga—Zn oxide film by a plasma CVD method.

As can be seen from FIG. 28, the temperature dependence of resistivityof the oxide conductor film (OC) is lower than the temperaturedependence of resistivity of the oxide semiconductor film (OS).Typically, the range of variation of resistivity of the oxide conductorfilm (OC) at temperatures from 80 K to 290 K is from more than −20% toless than +20%. Alternatively, the range of variation of resistivity attemperatures from 150 K to 250 K is from more than −10% to less than+10%. In other words, the oxide conductor is a degenerate semiconductorand it is suggested that the conduction band edge agrees with orsubstantially agrees with the Fermi level. Therefore, the oxideconductor film can be used as one electrode of the capacitor 790 a.

In addition, the display device 700 illustrated in FIG. 23 includes aliquid crystal element 775. The liquid crystal element 775 includes theconductive film 772, a conductive film 774, and a liquid crystal layer776. The conductive film 774 is provided on the second substrate 705side and functions as a counter electrode. The display device 700 inFIG. 23 is capable of displaying an image in such a manner thattransmission or non-transmission is controlled by change in thealignment state of the liquid crystal layer 776 depending on a voltageapplied to the conductive film 772 and the conductive film 774.

The conductive film 772 is connected to the conductive films functioningas a source electrode and a drain electrode of the transistor 750. Theconductive film 772 is formed over the insulating film 768 to functionas a pixel electrode, i.e., one electrode of the display element.

The conductive film 772 can be formed using a material and a methodsimilar to those of the conductive films 120, 120 a, and 120 b describedin the above embodiment.

Although not illustrated in FIG. 23, an alignment film may be providedon a side of the conductive film 772 in contact with the liquid crystallayer 776 and on a side of the conductive film 774 in contact with theliquid crystal layer 776. Although not illustrated in FIG. 23, anoptical member (an optical substrate) and the like such as a polarizingmember, a retardation member, or an anti-reflection member may beprovided as appropriate. For example, circular polarization may beemployed by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a sidelight, or the like may be used as a lightsource.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer-dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material exhibits acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on conditions.

Alternatively, in the case of employing a horizontal electric fieldmode, a liquid crystal exhibiting a blue phase for which an alignmentfilm is unnecessary may be used. A blue phase is one of liquid crystalphases, which is generated just before a cholesteric phase changes intoan isotropic phase while temperature of cholesteric liquid crystal isincreased. Since the blue phase appears only in a narrow temperaturerange, a liquid crystal composition in which several weight percent ormore of a chiral material is mixed is used for the liquid crystal layerin order to improve the temperature range. The liquid crystalcomposition which includes liquid crystal exhibiting a blue phase and achiral material has a short response time and optical isotropy, whichmakes the alignment process unneeded. In addition, the liquid crystalcomposition which includes liquid crystal exhibiting a blue phase and achiral material has a small viewing angle dependence. An alignment filmdoes not need to be provided and rubbing treatment is thus notnecessary; accordingly, electrostatic discharge damage caused by therubbing treatment can be prevented and defects and damage of the liquidcrystal display device in the manufacturing process can be reduced.

In the case where a liquid crystal element is used as the displayelement, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode,a fringe field switching (FFS) mode, an axially symmetric alignedmicro-cell (ASM) mode, an optical compensated birefringence (OCB) mode,a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquidcrystal (AFLC) mode, or the like can be used.

Furthermore, a normally black liquid crystal display device such as atransmissive liquid crystal display device utilizing a verticalalignment (VA) mode may also be used. There are some examples of avertical alignment mode; for example, a multi-domain vertical alignment(MVA) mode, a patterned vertical alignment (PVA) mode, an ASV mode, orthe like can be employed.

<Display Device Using Light-Emitting Element as Display Element>

The display device 700 illustrated in FIG. 24 includes the capacitor 790b. The capacitor 790 b includes a dielectric between a pair ofelectrodes. Specifically, a conductive film which is formed using stepsof forming the same conductive film as the conductive film functioningas a gate electrode of the transistor 750 is used as one electrode ofthe capacitor 790 b, and a conductive film functioning as a sourceelectrode or a drain electrode of the transistor 750 is used as theother electrode of the capacitor 790 b. Furthermore, an insulating filmfunctioning as a gate insulating film of the transistor 750 is used asthe dielectric between the pair of electrodes.

In FIG. 24, a planarization insulating film 770 is formed over theinsulating film 769.

The planarization insulating film 770 can be formed using aheat-resistant organic material, such as a polyimide resin, an acrylicresin, a polyimide amide resin, a benzocyclobutene resin, a polyamideresin, or an epoxy resin. Note that the planarization insulating film770 may be formed by stacking a plurality of insulating films formedfrom these materials. Alternatively, a structure without theplanarization insulating film 770 as illustrated in FIG. 23 may beemployed.

The display device 700 illustrated in FIG. 24 includes a light-emittingelement 782. The light-emitting element 782 includes a conductive film784, an EL layer 786, and a conductive film 788. The display device 700illustrated in FIG. 24 is capable of displaying an image by lightemission from the EL layer 786 of the light-emitting element 782.

The conductive film 784 is connected to the conductive films functioningas a source electrode and a drain electrode of the transistor 750. Theconductive film 784 is formed over the planarization insulating film 770to function as a pixel electrode, i.e., one electrode of the displayelement. A conductive film which transmits visible light or a conductivefilm which reflects visible light can be used for the conductive film784. The conductive film which transmits visible light can be formedusing a material including one kind selected from indium (In), zinc(Zn), and tin (Sn), for example. The conductive film which reflectsvisible light can be formed using a material including aluminum orsilver, for example.

In the display device 700 in FIG. 24, an insulating film 730 is providedover the planarization insulating film 770 and the conductive film 784.The insulating film 730 covers part of the conductive film 784. Notethat the light-emitting element 782 has a top emission structure.Therefore, the conductive film 788 has a light-transmitting property andtransmits light emitted from the EL layer 786. Although the top-emissionstructure is described as an example in this embodiment, one embodimentof the present invention is not limited thereto. A bottom-emissionstructure in which light is emitted to the conductive film 784 side, ora dual-emission structure in which light is emitted to both theconductive film 784 side and the conductive film 788 side may beemployed.

The coloring film 736 is provided to overlap with the light-emittingelement 782, and the light-blocking film 738 is provided to overlap withthe insulating film 730 and to be included in the lead wiring portion711 and in the source driver circuit portion 704. The coloring film 736and the light-blocking film 738 are covered with the insulating film734. A space between the light-emitting element 782 and the insulatingfilm 734 is filled with a sealing film 732. Although a structure withthe coloring film 736 is described as the structure of the displaydevice 700 in FIG. 24, the structure is not limited thereto. In the casewhere the EL layer 786 is formed so that different colors of light areemitted from different pixels, the coloring film 736 is not necessarilyprovided.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 4

In this embodiment, a display device that includes a semiconductordevice of one embodiment of the present invention will be described withreference to FIGS. 25A to 25C.

The display device illustrated in FIG. 25A includes a region includingpixels of display elements (hereinafter the region is referred to as apixel portion 502), a circuit portion being provided outside the pixelportion 502 and including a circuit for driving the pixels (hereinafterthe portion is referred to as a driver circuit portion 504), circuitseach having a function of protecting an element (hereinafter thecircuits are referred to as protection circuits 506), and a terminalportion 507. Note that the protection circuits 506 are not necessarilyprovided.

Part or the whole of the driver circuit portion 504 is preferably formedover a substrate over which the pixel portion 502 is formed, in whichcase the number of components and the number of terminals can bereduced. When part or the whole of the driver circuit portion 504 is notformed over the substrate over which the pixel portion 502 is formed,the part or the whole of the driver circuit portion 504 can be mountedby COG or tape automated bonding (TAB).

The pixel portion 502 includes a plurality of circuits for drivingdisplay elements arranged in X rows (X is a natural number of 2 or more)and Y columns (Y is a natural number of 2 or more) (hereinafter suchcircuits are referred to as pixel circuits 501). The driver circuitportion 504 includes driver circuits such as a circuit for supplying asignal (scan signal) to select a pixel (hereinafter the circuit isreferred to as a gate driver 504 a) and a circuit for supplying a signal(data signal) to drive a display element in a pixel (hereinafter thecircuit is referred to as a source driver 504 b).

The gate driver 504 a includes a shift register or the like. The gatedriver 504 a receives a signal for driving the shift register throughthe terminal portion 507 and outputs a signal. For example, the gatedriver 504 a receives a start pulse signal, a clock signal, or the likeand outputs a pulse signal. The gate driver 504 a has a function ofcontrolling the potentials of wirings supplied with scan signals(hereinafter such wirings are referred to as scan lines GL_1 to GL_X).Note that a plurality of gate drivers 504 a may be provided to controlthe scan lines GL_1 to GL_X separately. Alternatively, the gate driver504 a has a function of supplying an initialization signal. Withoutbeing limited thereto, the gate driver 504 a can supply another signal.

The source driver 504 b includes a shift register or the like. Thesource driver 504 b receives a signal (video signal) from which a datasignal is derived, as well as a signal for driving the shift register,through the terminal portion 507. The source driver 504 b has a functionof generating a data signal to be written to the pixel circuit 501 whichis based on the video signal. In addition, the source driver 504 b has afunction of controlling output of a data signal in response to a pulsesignal produced by input of a start pulse signal, a clock signal, or thelike. Furthermore, the source driver 504 b has a function of controllingthe potentials of wirings supplied with data signals (hereinafter suchwirings are referred to as data lines DL_1 to DL_Y). Alternatively, thesource driver 504 b has a function of supplying an initializationsignal. Without being limited thereto, the source driver 504 b cansupply another signal.

The source driver 504 b includes a plurality of analog switches, forexample. The source driver 504 b can output, as the data signals,signals obtained by time-dividing the video signal by sequentiallyturning on the plurality of analog switches. The source driver 504 b mayinclude a shift register or the like.

A pulse signal and a data signal are input to each of the plurality ofpixel circuits 501 through one of the plurality of scan lines GLsupplied with scan signals and one of the plurality of data lines DLsupplied with data signals, respectively. Writing and holding of thedata signal to and in each of the plurality of pixel circuits 501 arecontrolled by the gate driver 504 a. For example, to the pixel circuit501 in the m-th row and the n-th column (m is a natural number of lessthan or equal to X, and n is a natural number of less than or equal toY), a pulse signal is input from the gate driver 504 a through the scanline GL_m, and a data signal is input from the source driver 504 bthrough the data line DL_n in accordance with the potential of the scanline GL_m.

The protection circuit 506 in FIG. 25A is connected to, for example, thescan line GL between the gate driver 504 a and the pixel circuit 501.Alternatively, the protection circuit 506 is connected to the data lineDL between the source driver 504 b and the pixel circuit 501.Alternatively, the protection circuit 506 can be connected to a wiringbetween the gate driver 504 a and the terminal portion 507.Alternatively, the protection circuit 506 can be connected to a wiringbetween the source driver 504 b and the terminal portion 507. Note thatthe terminal portion 507 means a portion having terminals for inputtingpower, control signals, and video signals to the display device fromexternal circuits.

The protection circuit 506 is a circuit that electrically connects awiring connected to the protection circuit to another wiring when apotential out of a certain range is applied to the wiring connected tothe protection circuit.

As illustrated in FIG. 25A, the protection circuits 506 are provided forthe pixel portion 502 and the driver circuit portion 504, so that theresistance of the display device to overcurrent generated byelectrostatic discharge (ESD) or the like can be improved. Note that theconfiguration of the protection circuits 506 is not limited thereto, andfor example, the protection circuit 506 may be configured to beconnected to the gate driver 504 a or the protection circuit 506 may beconfigured to be connected to the source driver 504 b. Alternatively,the protection circuit 506 may be configured to be connected to theterminal portion 507.

In FIG. 25A, an example in which the driver circuit portion 504 includesthe gate driver 504 a and the source driver 504 b is shown; however, thestructure is not limited thereto. For example, only the gate driver 504a may be formed and a separately prepared substrate where a sourcedriver circuit is formed (e.g., a driver circuit substrate formed with asingle crystal semiconductor film or a polycrystalline semiconductorfilm) may be mounted.

Each of the plurality of pixel circuits 501 in FIG. 25A can have thestructure illustrated in FIG. 25B, for example.

The pixel circuit 501 illustrated in FIG. 25B includes a liquid crystalelement 570, a transistor 550, and a capacitor 560. As the transistor550, any of the transistors described in the above embodiment, forexample, can be used.

The potential of one of a pair of electrodes of the liquid crystalelement 570 is set in accordance with the specifications of the pixelcircuit 501 as appropriate. The alignment state of the liquid crystalelement 570 depends on written data. A common potential may be suppliedto one of the pair of electrodes of the liquid crystal element 570included in each of the plurality of pixel circuits 501. Furthermore,the potential supplied to one of the pair of electrodes of the liquidcrystal element 570 in the pixel circuit 501 in one row may be differentfrom the potential supplied to one of the pair of electrodes of theliquid crystal element 570 in the pixel circuit 501 in another row.

As examples of a driving method of the display device including theliquid crystal element 570, any of the following modes can be given: aTN mode, an STN mode, a VA mode, an axially symmetric aligned micro-cell(ASM) mode, an optical compensated birefringence (OCB) mode, aferroelectric liquid crystal (FLC) mode, an antiferroelectric liquidcrystal (AFLC) mode, an MVA mode, a patterned vertical alignment (PVA)mode, an IPS mode, an FFS mode, a transverse bend alignment (TBA) mode,and the like. Other examples of the driving method of the display deviceinclude an electrically controlled birefringence (ECB) mode, apolymer-dispersed liquid crystal (PDLC) mode, a polymer network liquidcrystal (PNLC) mode, and a guest-host mode. Note that one embodiment ofthe present invention is not limited to these examples, and variousliquid crystal elements and driving methods can be applied to the liquidcrystal element and the driving method thereof.

In the pixel circuit 501 in the m-th row and the n-th column, one of asource electrode and a drain electrode of the transistor 550 iselectrically connected to the data line DL_n, and the other iselectrically connected to the other of the pair of electrodes of theliquid crystal element 570. A gate electrode of the transistor 550 iselectrically connected to the scan line GL_m. The transistor 550 has afunction of controlling whether to write a data signal by being turnedon or off.

One of a pair of electrodes of the capacitor 560 is electricallyconnected to a wiring to which a potential is supplied (hereinafterreferred to as a potential supply line VL), and the other iselectrically connected to the other of the pair of electrodes of theliquid crystal element 570. The potential of the potential supply lineVL is set in accordance with the specifications of the pixel circuit 501as appropriate. The capacitor 560 functions as a storage capacitor forstoring written data.

For example, in the display device including the pixel circuit 501 inFIG. 25B, the pixel circuits 501 are sequentially selected row by row bythe gate driver 504 a illustrated in FIG. 25A, whereby the transistors550 are turned on and a data signal is written.

When the transistors 550 are turned off, the pixel circuits 501 in whichthe data has been written are brought into a holding state. Thisoperation is sequentially performed row by row; thus, an image can bedisplayed.

Alternatively, each of the plurality of pixel circuits 501 in FIG. 25Acan have the structure illustrated in FIG. 25C, for example.

The pixel circuit 501 illustrated in FIG. 25C includes a transistor 552,a transistor 554, a capacitor 562, and a light-emitting element 572. Anyof the transistors described in the above embodiment, for example, canbe used as one or both of the transistors 552 and 554.

One of a source electrode and a drain electrode of the transistor 552 iselectrically connected to a wiring to which a data signal is supplied(hereinafter referred to as a signal line DL_n). A gate electrode of thetransistor 552 is electrically connected to a wiring to which a gatesignal is supplied (hereinafter referred to as a scan line GL_m).

The transistor 552 has a function of controlling whether to write a datasignal by being turned on or off.

One of a pair of electrodes of the capacitor 562 is electricallyconnected to a wiring to which a potential is supplied (hereinafterreferred to as a potential supply line VL_a), and the other iselectrically connected to the other of a source electrode and a drainelectrode of the transistor 552.

The capacitor 562 functions as a storage capacitor for storing writtendata.

One of a source electrode and a drain electrode of the transistor 554 iselectrically connected to the potential supply line VL_a. Furthermore, agate electrode of the transistor 554 is electrically connected to theother of the source electrode and the drain electrode of the transistor552.

One of an anode and a cathode of the light-emitting element 572 iselectrically connected to a potential supply line VL_b, and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 554.

As the light-emitting element 572, an organic electroluminescent element(also referred to as an organic EL element) or the like can be used, forexample. Note that the light-emitting element 572 is not limited to anorganic EL element; an inorganic EL element including an inorganicmaterial may be used.

A high power supply potential VDD is supplied to one of the potentialsupply line VL_a and the potential supply line VL_b, and a low powersupply potential VSS is supplied to the other.

For example, in the display device including the pixel circuit 501 inFIG. 25C, the pixel circuits 501 are sequentially selected row by row bythe gate driver 504 a illustrated in FIG. 25A, whereby the transistors552 are turned on and a data signal is written.

When the transistors 552 are turned off, the pixel circuits 501 in whichthe data has been written are brought into a holding state. Furthermore,the amount of current flowing between the source electrode and the drainelectrode of the transistor 554 is controlled in accordance with thepotential of the written data signal. The light-emitting element 572emits light with a luminance corresponding to the amount of flowingcurrent. This operation is sequentially performed row by row; thus, animage can be displayed.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 5

In this embodiment, a display module and electronic devices that includea semiconductor device of one embodiment of the present invention willbe described with reference to FIG. 26 and FIGS. 27A to 27H.

In a display module 8000 illustrated in FIG. 26, a touch panel 8004connected to an FPC 8003, a display panel 8006 connected to an FPC 8005,a backlight unit 8007, a frame 8009, a printed board 8010, and a battery8011 are provided between an upper cover 8001 and a lower cover 8002.

The semiconductor device of one embodiment of the present invention canbe used for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and can be formed to overlap the display panel 8006. Acounter substrate (sealing substrate) of the display panel 8006 can havea touch panel function. A photosensor may be provided in each pixel ofthe display panel 8006 to form an optical touch panel.

The backlight unit 8007 includes a light source 8008. Note that althougha structure in which the light sources 8008 are provided over thebacklight unit 8007 is illustrated in FIG. 26, one embodiment of thepresent invention is not limited to this structure. For example, astructure in which the light source 8008 is provided at an end portionof the backlight unit 8007 and a light diffusion plate is furtherprovided may be employed. Note that the backlight unit 8007 need not beprovided in the case where a self-luminous light-emitting element suchas an organic EL element is used or in the case where a reflective panelor the like is employed.

The frame 8009 protects the display panel 8006 and also functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 may function asa radiator plate.

The printed board 8010 is provided with a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying power to the power supplycircuit, an external commercial power source or a power source using thebattery 8011 provided separately may be used. The battery 8011 can beomitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

FIGS. 27A to 27H illustrate electronic appliances. These electronicdevices can include a housing 9000, a display portion 9001, a speaker9003, an LED lamp 9004, operation keys 9005 (including a power switch oran operation switch), a connection terminal 9006, a sensor 9007 (asensor having a function of measuring or sensing force, displacement,position, speed, acceleration, angular velocity, rotational frequency,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, current, voltage, electric power,radiation, flow rate, humidity, gradient, oscillation, odor, or infraredray), a microphone 9008, and the like.

FIG. 27A illustrates a mobile computer that can include a switch 9009,an infrared port 9010, and the like in addition to the above components.FIG. 27B illustrates a portable image reproducing device (e.g., a DVDplayer) that is provided with a memory medium and can include a seconddisplay portion 9002, a memory medium reading portion 9011, and the likein addition to the above components. FIG. 27C illustrates a goggle-typedisplay that can include the second display portion 9002, a support9012, an earphone 9013, and the like in addition to the abovecomponents. FIG. 27D illustrates a portable game machine that caninclude the memory medium reading portion 9011 and the like in additionto the above components. FIG. 27E illustrates a digital camera that hasa television reception function and can include an antenna 9014, ashutter button 9015, an image receiving portion 9016, and the like inaddition to the above components. FIG. 27F illustrates a portable gamemachine that can include the second display portion 9002, the memorymedium reading portion 9011, and the like in addition to the abovecomponents. FIG. 27G illustrates a television receiver that can includea tuner, an image processing portion, and the like in addition to theabove components. FIG. 27H illustrates a portable television receiverthat can include a charger 9017 capable of transmitting and receivingsignals, and the like in addition to the above components.

The electronic devices illustrated in FIGS. 27A to 27H can have avariety of functions, for example, a function of displaying a variety ofdata (a still image, a moving image, a text image, and the like) on thedisplay portion, a touch panel function, a function of displaying acalendar, date, time, and the like, a function of controlling a processwith a variety of software (programs), a wireless communicationfunction, a function of being connected to a variety of computernetworks with a wireless communication function, a function oftransmitting and receiving a variety of data with a wirelesscommunication function, a function of reading a program or data storedin a memory medium and displaying the program or data on the displayportion, and the like. Furthermore, the electronic device including aplurality of display portions can have a function of displaying imagedata mainly on one display portion while displaying text data on anotherdisplay portion, a function of displaying a three-dimensional image bydisplaying images on a plurality of display portions with a parallaxtaken into account, or the like. Furthermore, the electronic deviceincluding an image receiving portion can have a function of shooting astill image, a function of taking a moving image, a function ofautomatically or manually correcting a shot image, a function of storinga shot image in a memory medium (an external memory medium or a memorymedium incorporated in the camera), a function of displaying a shotimage on the display portion, or the like. Note that functions that canbe provided for the electronic devices illustrated in FIGS. 27A to 27Hare not limited to those described above, and the electronic devices canhave a variety of functions.

The electronic devices described in this embodiment each include thedisplay portion for displaying some sort of data. Note that thesemiconductor device of one embodiment of the present invention can alsobe used for an electronic device that does not have a display portion.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Example 1

In this example, evaluation results of an insulating film that can beused for the semiconductor device of one embodiment of the presentinvention will be described. Specifically, results of evaluating thenumber of ammonia molecules released by heating will be described.

First, a method for fabricating evaluated samples is described. Thefabricated samples are a sample A1, a sample A2, and a sample A3. Notethat the sample A1 is a sample for comparison, and the samples A2 and A3are each a sample of one embodiment of the present invention.

<Sample A1>

For the sample A1, a 100-nm-thick silicon nitride film was formed over aglass substrate with a PECVD apparatus. As formation conditions of thesilicon nitride film, the substrate temperature was 350° C.; silane witha flow rate of 50 sccm, nitrogen with a flow rate of 5000 sccm, andammonia with a flow rate of 100 sccm were used as a source gas; thepressure in a treatment chamber was 100 Pa; and high-frequency power of1000 W (the power density of 1.6×10⁻¹ W/cm²) at 27.12 MHz was suppliedto parallel-plate electrodes.

<Sample A2>

For the sample A2, a 100-nm-thick silicon nitride film was formed over aglass substrate with a PECVD apparatus. As formation conditions of thesilicon nitride film, the substrate temperature was 350° C.; silane witha flow rate of 200 sccm, nitrogen with a flow rate of 2000 sccm, andammonia with a flow rate of 100 sccm were used as a source gas; thepressure in a treatment chamber was 100 Pa; and high-frequency power of2000 W (the power density of 3.2×10⁻¹ W/cm²) at 27.12 MHz was suppliedto parallel-plate electrodes.

<Sample A3>

For the sample A3, a 100-nm-thick silicon nitride film was formed over aglass substrate with a PECVD apparatus. As formation conditions of thesilicon nitride film, the substrate temperature was 350° C.; silane witha flow rate of 200 sccm and nitrogen with a flow rate of 5000 sccm wereused as a source gas; the pressure in a treatment chamber was 100 Pa;and high-frequency power of 2000 W (the power density of 3.2×10⁻¹ W/cm²)at 27.12 MHz was supplied to parallel-plate electrodes.

Next, thermal desorption spectroscopy (TDS) analyses were performed onthe samples A1 to A3 fabricated as described above. In each of thesamples, the glass substrate was heated at a temperature higher than orequal to 65° C. and lower than or equal to 610° C.

The peaks of the curves shown in the results obtained from TDS appeardue to release of atoms or molecules contained in the analyzed samples(in this example, the samples A1 to A3) to the outside. The total numberof the atoms or molecules released to the outside corresponds to theintegral value of the peak. Thus, with the degree of the peak intensity,the number of the atoms or molecules contained in the silicon nitridefilm can be evaluated.

FIG. 29 shows the results of the TDS analyses on the samples A1 to A3.Note that FIG. 29 is a graph showing the numbers of ammonia moleculesreleased in the samples. The ammonia molecules were calculated from theintegral values of curve peaks that showed the amount of a released gaswhich had a M/z of 17, typically ammonia molecules, which was observedin TDS analysis.

According to FIG. 29, the number of ammonia molecules released in thesample A1 was 3.8×10¹⁵ molecules/cm³, that in the sample A2 was 5.2×10¹³molecules/cm³, and that in the sample A3 was 7.6×10¹³ molecules/cm³.

The structure described in this example can be used in appropriatecombination with any of the structures described in the otherembodiments and examples.

Example 2

In this example, evaluation results of conductive films and insulatingfilms that can be used for the semiconductor device of one embodiment ofthe present invention will be described. Observation results of theconductive films and the insulating films with the optical microscopewill be described in detail.

First, a method for fabricating evaluated samples is described. Thefabricated samples are a sample B1, a sample B2, and a sample B3. Notethat the sample B1 is a sample for comparison, the sample B2 is a sampleof one embodiment of the present invention, and the sample B3 is asample for comparison. FIG. 30 is a top view common to the samples B1 toB3. Description will be given below with reference to FIG. 30.

<Sample B1>

For the sample B1, a first conductive film 802 was formed over a glasssubstrate. The first conductive film 802 had a stacked-layer structureof three layers of a 50-nm thick tungsten film, a 400-nm-thick aluminumfilm, and a 100-nm-thick titanium film. Note that the first conductivefilm 802 was formed with a sputtering apparatus. Then, a mask was formedover the first conductive film 802 by a lithography process and thenprocessed with a dry etching apparatus, and the first conductive film802 was processed into desired shapes (first conductive films 802 a and802 b in FIG. 30).

Next, a first insulating film was formed over the first conductive film802. The first insulating film had a stacked-layer structure of twolayers of a 50-nm-thick first silicon oxynitride film and a 400-nm-thicksecond silicon oxynitride film. As formation conditions of the firstsilicon oxynitride film, the substrate temperature was 220° C.; silanewith a flow rate of 50 sccm and dinitrogen monoxide with a flow rate of2000 sccm were used as a source gas; the pressure in a treatment chamberwas 20 Pa; and high-frequency power of 100 W (the power density of1.6×10⁻² W/cm²) at 13.56 MHz was supplied to parallel-plate electrodes.As formation conditions of the second silicon oxynitride film, thesubstrate temperature was 220° C.; silane with a flow rate of 160 sccmand dinitrogen monoxide with a flow rate of 2000 sccm were used as asource gas; the pressure in a treatment chamber was 200 Pa; andhigh-frequency power of 1500 W (the power density of 2.4×10⁻¹ W/cm²) at13.56 MHz was supplied to parallel-plate electrodes.

Next, heat treatment was performed at 350° C. in a mixed gas atmosphereof a nitrogen gas and an oxygen gas for 1 hour.

Next, an opening 806 was formed in the first insulating film. Theopening 806 was formed to reach the first conductive films 802 a and 802b. Note that a plurality of openings 806 (four openings 806 in FIG. 30)were formed.

Next, a second conductive film 804 was formed over the first insulatingfilm so as to cover the openings 806. As the second conductive film 804,a 100-nm-thick indium tin oxide film to which silicon oxide was addedwas formed. As formation conditions of the indium tin oxide film towhich silicon oxide was added, the substrate temperature was roomtemperature; argon with a flow rate of 72 sccm and oxygen with a flowrate of 5 sccm were used as a deposition gas; the pressure in atreatment chamber was 0.15 Pa; and DC power of 3200 W was supplied to asputtering target (In₂O₃:SnO₂:SiO₂=85:10:5 [wt %]). Then, a mask wasformed over the second conductive film 804 by a lithography process andthen processed with a dry etching apparatus, and the second conductivefilm 804 was processed into a desired shape. Note that the secondconductive film 804 had a comb-like electrode shape as illustrated inFIG. 30. As the size of the comb-like electrode, L/W was set to 24436.55mm/5 nm. The two-dot chain line is an ellipsis indicating that thesecond conductive film 804 is not fully illustrated in the L lengthdirection. One end of the comb-like electrode was electrically connectedto the first conductive film 802 a and the other end of the comb-likeelectrode was electrically connected to the first conductive film 802 b.

Next, a third insulating film was formed over the second conductive film804. The third insulating film was formed under the same conditions asthe silicon nitride film of the sample A1 described in the aboveexample.

<Sample B2>

For the sample B2, a first conductive film 802 (first conductive films802 a and 80 b), a first insulating film over the first conductive film802 (first conductive films 802 a and 802 b), and a second conductivefilm 804 over the first insulating film were formed over a glasssubstrate. Note that the first conductive film 802 (first conductivefilms 802 a and 802 b), the first insulating film, and the secondconductive film 804 were formed under the same conditions as the sampleB1 described above using the same material.

Next, a third insulating film was formed over the second conductivefilm. The third insulating film was formed under the same conditions asthe silicon nitride film of the sample A3 described in the aboveexample.

<Sample B3>

For the sample B3, the first conductive film 802 (first conductive films802 a and 802 b), the first insulating film over the first conductivefilm 802 (first conductive films 802 a and 802 b), and the secondconductive film 804 over the first insulating film were formed over aglass substrate. Note that the first conductive film 802 (firstconductive films 802 a and 802 b), the first insulating film, and thesecond conductive film 804 were formed under the same conditions as thesample B1 described above using the same material. Note that in thesample B3, the third insulating film is not formed over the secondconductive film 804.

Next, the appearances of the samples B1 and B2 fabricated as describeabove were observed with an optical microscope.

FIGS. 31A and 31B show the appearance of the sample B1 and theappearance of the sample B2, respectively, which were observed with anoptical microscope. Note that FIG. 31A shows the result of the sampleB1, and FIG. 31B shows the result of the sample B2.

According to the results shown in FIGS. 31A and 31B, many defects inappearance were found in the sample B1; however, no defect in appearancewas found in the sample B2. Note that it was found that most of thedefects in appearance were caused by alteration of the second conductivefilm 804. This is probably because the conditions for formation of thesilicon nitride films used as the third insulating films in the samplesB1 and B2 were different. The silicon nitride film used as the thirdinsulating film in the sample B1 is an insulating film which releasesammonia molecules in excess of 1×10¹⁵ molecules/cm³ as described inExample 1, and the silicon nitride film used as the third insulatingfilm in the sample B2 is an insulating film which releases ammoniamolecules of less than or equal to 1×10¹⁵ molecules/cm³ as described inExample 1. Although not shown in FIGS. 31A and 31B, there was no defectin appearance because the third insulating film was not formed in thesample B3. Therefore, it is indicated that the second conductive film804 is altered when a number of ammonia molecules are released from thethird insulating film.

Next, a stress test under high temperature and high humidity wasperformed on the samples B2 and B3 fabricated as described above. Asconditions of the stress test under high temperature and high humidity,the temperature and humidity of the evaluation environment were 60° C.and 95%, respectively. A voltage of 15 V was applied to the firstconductive films 802 a and 802 b and the second conductive film 804 for12 hours. Note that as a method for applying voltage to the secondconductive film 804, voltage was applied from the outside in such amanner that 15 V was applied to the first conductive film 802 a in FIG.30 and the first conductive film 802 b was fixed to the 0 V.

Next, the appearances of the samples B2 and B3 subjected to the abovestress test under high temperature and high humidity were observed withan optical microscope.

FIGS. 32A and 32B show the appearance of the sample B2 and theappearance of the sample B3, respectively, which were observed with anoptical microscope. Note that FIG. 32A shows the result of the sampleB2, and FIG. 32B shows the result of the sample B3.

According to the results shown in FIGS. 32A and 32B, there was lessdefects in the appearance of the sample B2 in which the third insulatingfilm was formed over the second conductive film. On the other hand,corrosion of the first conductive film and the second conductive filmwas found in the sample B3 in which the third insulating film was notformed over the second conductive film.

As described above, it was found that corrosion of the first conductivefilm 802 and the second conductive film 804 which were subjected to thestress test under high temperature and high humidity was able to besuppressed when a silicon nitride film which would release ammoniamolecules of less than or equal to 1×10¹⁵ molecules/cm³ in TDS analysiswas formed as the third insulating film over the second conductive film,here, the indium tin oxide film to which silicon oxide was added.

The structure described in this example can be used in appropriatecombination with any of the structures described in the otherembodiments and examples.

This application is based on Japanese Patent Application serial No.2014-057528 filed with Japan Patent Office on Mar. 20, 2014, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing a semiconductor devicecomprising the steps of: forming a first conductive film; forming afirst insulating film over the first conductive film; forming a secondconductive film over the first insulating film; forming a secondinsulating film over the second conductive film; forming an openingreaching the first conductive film in the first insulating film and thesecond insulating film; forming a third conductive film over the firstconductive film with the opening; forming a third insulating film overthe third conductive film, wherein a part of the third conductive filmis formed in the opening of the first insulating film and the secondinsulating film, wherein the third conductive film comprises one or moreof indium and oxygen, and wherein the third insulating film includessilicon and nitrogen, and the number of ammonia molecules released fromthe third insulating film is less than or equal to 1×10¹⁵ molecules/cm³as heated at a temperature higher than or equal to 65° C. lower than orequal to 610° C. by thermal desorption spectroscopy.
 2. The method formanufacturing the semiconductor device according to claim 1 wherein thethird insulating film is formed by plasma enhanced chemical vapordeposition without using an ammonia gas as deposition gas.
 3. The methodfor manufacturing the semiconductor device according to claim 2, whereinthe third insulating film is formed by the plasma enhanced chemicalvapor deposition using silane with a flow rate of 200 sccm and nitrogenwith a flow rate of 5000 sccm as deposition gas.
 4. The method formanufacturing the semiconductor device according to claim 1, wherein thethird insulating film is formed by a plasma enhanced chemical vapordeposition using silane with a flow rate of 200 sccm, nitrogen with aflow rate of 2000 sccm, and ammonia with a flow rate of 100 sccm asdeposition gas.
 5. The method for manufacturing the semiconductor deviceaccording to claim 1, wherein the third conductive film is formed bysputtering using argon with a flow rate of 72 sccm and oxygen with aflow rate of 5 sccm as a sputtering gas, and sputtering target(In₂O₃:SnO₂:SiO₂=85:10:5 [wt %]).
 6. The method for manufacturing thesemiconductor device according to claim 1, further comprising the stepof forming an oxide semiconductor film over the first insulating film,wherein the oxide semiconductor film includes oxygen, In, Zn, and M (Mis Ti, Ga, Y, Zr, La, Ce, Nd, or Hf).
 7. The method for manufacturingthe semiconductor device according to claim 1, wherein the thirdinsulating film is a single-layer structure.
 8. A method formanufacturing a semiconductor device comprising the steps of: forming afirst conductive film; forming a first insulating film over the firstconductive film; forming an oxide semiconductor film over the firstinsulating film; forming a pair of second conductive films electricallyconnected to the oxide semiconductor film; forming a second insulatingfilm over the oxide semiconductor film and the pair of second conductivefilms; forming an opening reaching the first conductive film in thefirst insulating film and the second insulating film; forming a thirdconductive film over the first conductive film with the opening; forminga third insulating film over the third conductive film, wherein a partof the third conductive film is formed in the opening of the firstinsulating film and the second insulating film, wherein the thirdconductive film comprises one or more of indium and oxygen, and whereinthe third insulating film includes silicon and nitrogen, and the numberof ammonia molecules released from the third insulating film is lessthan or equal to 1×10¹⁵ molecules/cm³ as heated at a temperature higherthan or equal to 65° C. lower than or equal to 610° C. by thermaldesorption spectroscopy.
 9. The method for manufacturing thesemiconductor device according to claim 8, wherein the third insulatingfilm is formed by plasma enhanced chemical vapor deposition withoutusing an ammonia gas as deposition gas.
 10. The method for manufacturingthe semiconductor device according to claim 9, wherein the thirdinsulating film is formed by the plasma enhanced chemical vapordeposition using silane with a flow rate of 200 sccm and nitrogen with aflow rate of 5000 sccm as deposition gas.
 11. The method formanufacturing the semiconductor device according to claim 8, wherein thethird insulating film is formed by a plasma enhanced chemical vapordeposition using silane with a flow rate of 200 sccm, nitrogen with aflow rate of 2000 sccm, and ammonia with a flow rate of 100 sccm asdeposition gas.
 12. The method for manufacturing the semiconductordevice according to claim 8, wherein the third conductive film is formedby sputtering using argon with a flow rate of 72 sccm and oxygen with aflow rate of 5 sccm as a sputtering gas, and sputtering target(In₂O₃:SnO₂:SiO₂=85:10:5 [wt %]).
 13. The method for manufacturing thesemiconductor device according to claim 8, further comprising the stepof forming an oxide semiconductor film over the first insulating film,wherein the oxide semiconductor film includes oxygen, In, Zn, and M (Mis Ti, Ga, Y, Zr, La, Ce, Nd, or Hf).
 14. The method for manufacturingthe semiconductor device according to claim 8, wherein the thirdinsulating film is a single-layer structure.